Improvements in or relating to Automatic Controllers for Air and Gas Compressors.
Year: 1899

page_text: Date of Application, 23rd Oct., 1899
Complete Specification Left, 27th Feb., 1900-Accepted, 21st Apr., 1900
PROVISIONAL SPECIFICATION.
Improvements in or relating to Automatic Controllers for Air and Gas
Compressors.
A communication from NIELS ANTON CHRISTENSEN, Mechanical Engineer, of
-Milwaukee, County of Milwaukee, State of Wisconsin, U.S.A.
I, ALFRED JULIUS BOULT, of 111, Hatton Garden, in the County of London,
Chartered Patent Agent, do hereby declare the nature of this invention
to be as follows:-
This invention relates to electrically actuated compressors.
Its main object is to maintain pressure produced by such compressors within
certain limits by automatically stopping and starting the compressor motor when
the pressure reaches the higher limit and falls to the lower limit.
It consists in certain novel features of construction and arrangement and in
the combination of parts hereinafter particularly described.
In carrying out the invention two magnets, preferably of the solenoid type,
are arranged in line with each other, a core projecting loosely into and being
movable axially in the coils of said magnets; a soft iron plate or armature being
mounted upon said core between the magnets.
The main switch, which constitutes a part of the apparatus and controls the
circuit of the compressor motor, consists of a number of fixed contacts, and a
The fixed contacts are mounted on
number of pairs of spring contact arms.
a block or bar of insulating material which is arranged transversely to the
magnets. The spring contact arms are hinged or pivotted in a line parallel with
the bar so as to be turned into and out of engagement with said fixed contacts
by the magnets, the switch being opened when the armature is attracted to one
The
magnet and closed when said armature is attracted to the other magnet.
first contact is connected with a binding screw or post, the next two contacts
with a common binding post, and the fourth contact with a third binding post
or screw. To an arm of the armature is attached an insulating strip or plate
of vulcanized fibre or other suitable insulating material, which is constructed and
arranged to be thrust between fixed and movable contacts of said switch as soon as
they are separated and thereby prevent or intercept arcs or sparks between said
contacts.
A pressure gauge, which has a fluid pressure connection with the compressor
or with a reservoir into which said compressor discharges is operated by a motor
This pressure gauge has a
which is governed by the automatic controller.
movable hand or index which is preferably made of insulating material and
Minimum and maximum pressure con-
carries a metallic or conducting contact.

page_text: tacts are arranged in the path of the movable contact and separately adjustable
towards and from it.
Fuses are also provided through which electrical connections with the contacts
of the pressure gauge are made.
Insulated contact springs are placed which normally engage with each other
and are arranged to be separated by an insulating strip carried by the armature
when the main switch is closed. One spring is electrically connected with a
binding post and the other spring with a second binding post. These contact
springs, with the insulating strip, constitute an auxiliary switch or circuit-
breaker which is arranged in and controls a shunt passing through the coil
of magnet.
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The controller may be placed in the compressor-motor circuit between said
motor and the source of current, when the circuit connections will be as follows:
The first binding post and that of one of the arms of the insulated contact
springs or auxiliary switch are connected' directly' with the source of current;
the third binding post is connected with one terminal of the motor; the other
terminal of said motor being connected with the ground; the second binding
post is connected with one terminal of the one magnet, the other terminal of
said magnet being connected with one terminal of the other magnet and through
a fuse with the movable contact of the pressure gauge; the other terminal of
this magnet being connected with the ground. The binding post of the auxiliary
switch or circuit controller is connected through a fuse with the minimum pressure
contact and the maximum pressure contact is connected directly with the ground.
When connected as above stated the "controller operates as follows:
Assuming that the pressure has reached the maximum limit for which the
controller is adjusted, and that the movable contact has been turned into engage-
ment with the maximum pressure contact, the contacts of the main switch will
be separated, thereby cutting out the motor and stopping the compressor, and
the contact springs will be released and allowed to engage with each other,
thereby closing the, shunt connection between the source of current and the
minimum pressure contact, through the coil of the second magnet. Under these
conditions the armature will remain attracted to the first magnet and the circuit
through both magnets will be open at the main switch and also at the pressure
gauge. As the pressure falls the hand will be turned gradually to say the left,
and when the minimum limit for which the controller is adjusted is reached the
hand contact will engage with the gauge contact, thereby closing the shunt
circuit through, the second magnet and energizing, said magnet. The circuit
through the first, magnet remaining open, the armature will be drawn by the
second, magnet to the right, thereby closing the main switch and thrusting the
insulating plate between and separating the contact springs in the shunt which
connects the minimum pressure contact through the magnet with the source of
current. The parts of the controller will now assume the positions in which
the current pass through the contacts of the main switch to the motor, and
thence to the ground, thereby starting the compressor which will remain in
operation.until the pressure reaches the maximum limit for which the controller
is, adjusted. A portion of the current will be shunted when the main switch is
thus closed,, from the second binding post, through the coils of
of both magnets and
thence to the ground; but this will not change the position of the armature which,
by reason of its closer, proximity to the second magnet, will be h
will be held by said
magnet while both magnets are energized. As the pressure rises the hand will be
turned gradually back to the right, and when the maximum limit is reached
the contact will engage, with the high pressure contact, thereby closing the direct
ground connection for the first magnet and short-circuiting the second magnet.
The first magnet remaining energized when the second magnet is thus de-ener
gized draws the armature back to the left into its original position, thereby
opening the main switch, stopping the compressor and closing the auxiliary
switch or circuit-controller in the shunt which connects the low pressure contact
1.
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page_text: with the source of current. Upon the separation of the contacts of the main
switch, the insulating. plate is thrust between them thereby preventing the
formation of arcs and consequent injury to the apparatus.
The controller may also be placed on the ground side of the motor, and when
so arranged the circuit connections may be made substantially as described except
that the connections with the source of current and ground will be reversed or
transposed.
The circuit connections may also be arranged in such a manner that the direct
electrical connection between the coils of the two magnets is omitted, one terminal
of the first magnet being connected with the maximum pressure contact, one
terminal of the second magnet with the switch spring and the other terminal of
said magnet with the low pressure contact, and the movable contact of the pressure
gauge being connected directly with the ground. In other respects the con-
nections are substantially the same as those already described. With this latter
arrangement of circuit connections the controller may be placed on the ground
or return side of the motor by simply inverting or reversing the connections with
the source of current and with the ground or return.
The controller herein before described in specially designed for use in con-
nection with air brake systems for electric railways employing independent motors
for supplying compressed air to operate the brakes, but it may be used for other
purposes.
Various changes in details of construction and arrangements of parts may be
made without changing the principle or materially affecting the operation of
the controller.
Dated this 92nd Joy of October 1800
BOULT, WADE & KILBURN,
Agents for the Applicant.
Improvements in or relating to Automatic Controllers for Air and Gas
Compressors.
A communication from NIELS ANTON CHRISTENSEN, Mechanical Engineer, of
359, Oakland Avenue, Milwaukee, County of Milwaukee, State of Wisconsin,
U.S.A.
I, ALFRED JULIUS BOULT, of 111, Hatton Garden, in the County of London,
Chartered Patent Agent, do hereby declare the nature of this invention
and in what manner the same is to be performed, to be particularly described and
ascertained in and by the following statement:-
This invention relates to electrically actuated compressors.
Its main object is to maintain the pressure within certain limits by automatically
stopping the compressor when the pressure reaches the higher limit and starting
it when the pressure falls to the lower limit.
It consists in certain novel features of construction and in the arrangement
and combination of parts of the controller hereinafter particularly described and
defined in the claims.
In the accompanying drawings like letters designate the same parts in the
several figures.
Fig. 1 is a front elevation of the mechanism of an automatic controller em-
bodying the invention, the enclosing case being shown in longitudinal section
and parts of the mechanism broken away; Fig 2 is a plan view of the same,
the cover being removed and certain parts of the mechanism broken away and

page_text: shown in horizontal section; Fig. 3 is a vertical longitudinal section on;, the
line 33, Fig. 2; Fig. 4 is a sectional view on the line 4 4, Fig. 2, of a part
of the main switch; Fig. 5 is a vertical cross-section on the line 5 5, Fig. 2;
Figs. 6 and are diagrams showing circuit connections of the controller, Fig. 6
representing the main switch open and Fig. 7 representing it closed; and Fig. 8
is a diagram showing a modified arrangement of the circuit connections.
Referring to Figs. 1 to 5 inclusive, A and B designate two solenoid magnets
nounted in line with each other upon a suitable base C. D is a core common to
both magnets, and E is a plate or armature carried by said core between said
magnets.
The main switch controlling the circuit of the compressor-motor, consists of a
number of fixed contacts F, F1, F2 and F3 and a number of pairs of spring
contact arms G, G¹, G2 and G³. The fixed contacts are counter-sunk in a bar H.
of insulating material supported above and transversely to the magnets upon
brackets or standards I I. The spring contact arms are mounted upon a rocking
bar J which is supported parallel with the bar H upon, and insulated from,
the brackets I. Each pair of these spring contact arms is arranged to be sprung
over and to embrace a tongue or raised portion of the corresponding fixed contact.
The arms G and G¹ are electrically connected with each other and insulated from
the arms G and G³, which are connected with each other in like manner.
rocking bar J is formed or provided with a depending arm and a horizontally
disposed arm curved downwardly and provided with a roller at its free end
which projects towards the bar. H. The contacts F, F1, F2 and F³ are provided
with binding screws f, f¹, f² and f³ which project through one side of the
insulating bar H. The binding screws f¹ and f² are electrically connected with a
common binding post ft.
The
The armature E is formed or provided with an upwardly extending arm e which
is adapted, when said armature is drawn towards the magnet B, to engage with
the arm of the rocking bar J and thereby turn the spring contact arms G etc.
of the switch down into engagement with the fixed contacts F etc.
To the arm e is attached a strip or plate K of vulcanized fiber or any suitable
insulating material, which is constructed and arranged to be thrust between the
fixed and movable contacts of the switch when they are separated and thereby
serves as a spark-arrester or intercepter to prevent the formation of arcs between
said contacts. Upon said intercepting strip or plate K is mounted a block L,
which is beveled or inclined at the end towards the insulating bar H, and is
attached with said strip or plate to the arm e in line with the roller 2, so that
when the armature E is moved towards the magnet A the arm will be lifted,
thereby raising the spring contact arms G etc. out of engagement with the fixed
contacts F etc.
M is a manometer or pressure gauge containing a Bourdon spring-tube N,
which is attached to and communicates at one end with a fluid-pressure supply
pipe leading from the compressor or a reservoir into which the compressor
discharges. This fluid-pressure supply connection is formed or provided, for
convenience in connecting the controller under varying conditions, with two
branches o and o¹, one leading through the end and the other through the side
of the case or base C. Whichever of these branches is not used in connecting the
controller with the compressor or reservoir is closed with a plug or by other
suitable means. The free end of the spring-tube is connected by links with a
segment gear n, which engages with a pinion p on the arbor of a hand or index P,
preferably made of non-conducting material and carrying a metallic or con-
ducting contact p¹. The connections above described multiply or increase the
movement imparted by the spring-tube N to the contact pl.
Q is a dial or face-plate through which the arbor of the hand projects at its
front end and over which the hand is arranged to vibrate. This dial or face-
plate may be provided with a scale, as indicated in Fig. 1, to facilitate the adjust-
ment of the controller for maximum and minimum pressures, as hereinafter

page_text: explained. R and S are minimum and maximum pressure contacts adjustably
secured in a curved slot q of the face-plate or dial on opposite sides of the movable
contact carried by the hand P. is a back plate or guard arranged behind
and opposite the slot q in the dial for holding the inner parts of the contacts R
and S in said slot when they are released for adjustment. The parts of the
pressure gauge above enumerated are enclosed in a suitable case which is pro-
vided on the front side with a removable cover m. TT are fuses through
which the electrical connections with the contacts of the pressure gauge are made.
They are mounted upon an insulating block T² which is attached to the back of
the case of the pressure gauge.
U and U are insulated contact springs attached to the upper and under sides
of a longitudinal bar U2 carried by the rear bracket I. They are constructed
and arranged to normally engage with each other and to be separated by an
insulating strip K¹ which is carried by the armature E and may be formed with
the spark-arresting strip or plate K. The upper spring U is electrically connected
with a binding post, and the lower spring U¹ is connected in like manner
with a binding post .
a and a designate the terminals of the coil of magnet A, and b and b¹ designate
the terminals of the coil of magnet B.
The controller may be placed in the circuit between the compressor-motor and
the source of current, the circuit connections when the controller is so arranged
with relation to the motor and source of current, being shown by the diagrams
in Figs. 6 and 7, according to which the binding posts fand are connected with
the source of current; the binding post fa is connected with one terminal of the
compressor-motor, the other terminal of said motor being connected with the
ground; the binding post f¹ is connected with the terminal a of magnet A, the
other terminal a' of said magnet being connected with the terminal 6 of magnet B;
the other terminal of magnet B is connected with the ground; the terminal a¹
of magnet A and the terminal of magnet B are connected through the fuse T
with the movable contact 7 of the pressure gauge; the binding post is con-
nected through the fuse T with the minimum pressure contact R, the contact
springs U, U and the insulating strip K¹ thus constituting a circuit breaker
or controller in a shunt between the source of current and the minimum pressure
contact, and the maximum pressure contact $ is connected with the ground
or return.
When connected as above explained the controller operates as follows:
Assuming that the pressure has reached the maximum limit and that the
movable contact p¹ has been turned into engagement with the maximum pressure
contact S, as shown in Fig. 6, the contacts of the main switch G, F etc. will
be separated, thereby cutting off current from the motor and stopping the com-
pressor, and the contact springs U and U₁ will be released and allowed to engage
with each other, thereby closing the shunt connection between the source of
current and the minimum pressure contact R. Under these conditions the
armature E will remain at the left next to the magnet A, and the circuit through
both magnets will be broken. As the pressure falls the hand P will be turned
gradually to the left, and when the minimum limit for which the controller is
adjusted is reached the contact p will be brought into engagement with the
contact R, thereby closing the shunt circuit through magnet B and energizing
said magnet.
The circuit through magnet A remaining open, the armature E
will be drawn by the magnet B to the right, thereby closing the main switch
and separating the contacts of the circuit controller in the shunt which connects
the minimum pressure contact with the source of current. The parts of the
controller will now occupy the positions and the circuits will be established,
as shown in Fig. 7, current passing through the contacts of the main switch to
the motor, thence to the ground, thereby starting the compressor, which will
remain in operation until the maximum limit for which the controller is adjusted
is reached.
A portion of the current will be shunted, when the main switch is

page_text: thus closed, from the binding post f*, through the coils of the two magnets
and thence to the ground; but this will not change the position of the armature E,
which by reason of its closer proximity with the magnet B will be held by said
magnet while both magnets remain energized. As the pressure rises the hand P
will be turned gradually back to the right, and when the maximum limit is
reached, the contact will engage with the maximum pressure contact S.
thereby closing the direct ground connection for the magnet A through said
movable and maximum pressure contacts and short circuiting the magnet B.
The magnet A remaining energized while the magnet B is thus de-energized
draws the armature E back to the left into the position in which it is shown in
Fig. 6, thereby opening the main switch, stopping the compressor and closing the
circuit controller in the shunt connecting the low pressure contact R with the
source of current. Immediately after the beveled or inclined block L lifts the
arm j, turning the contact springs G etc. up out of engagement with the
fixed contacts etc., the spark-arresting plate K is thrust between said arms and
contacts, thereby preventing the formation of arcs between them and consequent
injury to the apparatus.
The controller may also be placed on the ground side of the motor by simply
reversing the circuit connections, making the connection with the source of
current at the point designated "ground in Figs. 6 and 7, and making the
ground connection at the point designated source of current" in the same.
figures.
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:
The circuit connections may also be arranged as shown in Fig. 8, in which
the direct electrical connection between the two magnets A and B is omitted, the
terminal a¹ of magnet A being connected with the maximum pressure contact S,
one terminal of the magnet B with the contact spring U₁, and the other terminal
of said magnet with the low pressure contact R, and the movable contact p¹ of
the pressure gauge being connected directly with the ground. In other respects
the connections are substantially the same as those shown in Figs. and 7.
With this arrangement of circuit connections the controller may be placed on
the ground or return side of the motor by simply inverting or reversing the con-
nections with the source of current and with the ground or return.
The controller herein shown and described is specially designed for use in
connection with air brake systems for electric railways employing independent
motors for supplying compressed air to operate the brakes, but it may be used
for other purposes.
Various changes in details of construction and arrangement of parts may
be made within the spirit and intended scope of the invention.
Having now particularly described and ascertained the nature of this said
invention, as communicated to me by my foreign correspondent and in what
manner the same is to be performed, I declare that what I claim is:
1. In an automatic controller for electric compressors the combination of a
movable contact having a fluid pressure actuating connection, maximum and
minimum pressure contacts arranged to co-operate with said movable contact,
a switch in the compressor-motor circuit, two magnets having circuit connections
so arranged that when said movable contact engages the maximum pressura
contact the switch will be opened and the compressor stopped and when it
engages the minimum pressure contact the switch will be closed and the com-
pressor started, and a spark-arrester arranged to be thrust between the contacts of
said switch as soon as they are separated, substantially as and for the purposes
set forth.
2. In an automatic controller for electric compressors the combination of a
movable contact having a fluid-pressure actuating connection, maximum and
minimum pressure contacts arranged to co-operate with said movable contact,
a switch in the compressor-motor circuit, two magnets connected in series with
each other and with said switch and arranged to open and close the same, the

page_text: coil of one of said magnets being connected at one end with the ground or
return, and at the other end with said movable contact, the maximum pressure
contact being connected with the ground or return and the minimum pressure
contact with the source of current, and a circuit controller in the circuit con-
nection of the minimum pressure contact arranged to be closed when the main
switch is opened and vice versa, substantially as and for the purposes set forth.
3. In an automatic controller for electric compressors the combination of a
movable contact, a spring tube having a fluid pressure actuating connection and
connected with and arranged to actuate said movable contact, maximum and
minimum pressure contacts arranged to co-operate with said movable contact,
a switch in the compressor-motor circuit, and two magnets having circuit con-
nections so arranged that when said movable contact engages the maximum
pressure contact the switch will be opened and the compressor stopped, and when
it engages the minimum pressure contact the switch will be closed and the com-
pressor started, substantially as and for the purposes set forth.
4. In an automatic controller for electric compressors the combination of a
hand or index carrying an electrical contact, a spring-tube having a fluid-
actuating connection and an actuating connection with said hand or index;" a
plate or dial having a curved slot, maximum and minimum pressure contacts
adjustably secured'in said slot on opposite sides of and in the path of the movable
contact carried by said.hand or index, a switch in the compressor-motor circuit,
and two magnets having circuit connections so arranged that when said movable
contact engages the maximum "pressure contact the switch will be opened and
the compressor stopped, and when it engages the minimum pressure contact the
switch will be closed and the compressor started; substantially as and for the
purposes set forth...„.
In an automatic controller for electric compressors the combination of a
movable contact having a fluid-pressure actuating connection, maximum and
minimum pressure contacts arranged to co-operate with said movable contact, a
switch" in the compressor-motor circuit comprising fixed insulated contacts and
a series of pivoted spring contact arms adapted to be turned into and out of
engagement with said fixed contacts, two magnets adapted to open and close said
switch, and a spark-arrester arranged to be thrust by said magnets between the
contacts of said switch as soon as they are separated, said magnets having circuit
connections so arranged that the switch will be opened and the compressor
stopped when the maximum pressure' contact is engaged by the movable contact
and said switch will be closed and the compressor started when the minimum
pressure contact is engaged by said movable contact, substantially-as and for the
purposes set forth.
31
- 1. -¦...
as
6. In an automatic controller for electric compressors the combination of a
movable contact having a fluid-pressure actuating connection, maximum and
minimum pressure contacts arranged to co-operate with said movable contact, a
switch in the compressor-motor circuit comprising a number of fixed. insulated
contacts, a rocking bar having a series of spring contact arms mounted thereon
and adapted to be turned into and out of engagement with said fixed contacts,
two magnets the armature of which is adapted to close said switch, an incline
carried by said armature in position by engagement with an arm of said rocking
bar to open said switch, and a spark-arresting plate carried by said armature. in
position to be thrust between the contacts of the switch as soon as they are
separated by said incline, substantially as and for the purposes set forth.......
...... ".
7. In an automatic controller for electric compressors the combination of a
movable contact having a fluid-pressure actuating connection; maximum and
minimum pressure contacts arranged to co-operate with said movable contact,
a main switch in the compressor-motor circuit, two magnets for operating said
switch connected in series with each other and with said switch, the coil of one of
said magnets being connected at one end with the ground or return and at the

page_text: other end with said movable contact, the maximum pressure contact being con-
nected with the ground and the minimum pressure contact having a shunt con-
nection with the source of current, a circuit controller in said shunt connection
consisting of two normally engaging springs or contacts, and an insulating strip
operated by said magnets to separate said springs when the main switch is closed
and to release them and permit them to come together when the main switch is
opened, substantially as and for the purposes set forth.
S. In an automatic circuit controller for electric compressors the combination
of a movable contact having a fluid-pressure actuating connection, maximum and
minimum pressure contacts arranged to co-operate with said movable contact,
a switch in the compressor motor circuit, two magnets arranged to open and close
said switch and connected in series with each other, the coil of one of said magnets
being connected at one end with the ground or return and at the other with
said movable contact, the maximum pressure contact being connected with the
ground or return and the minimum pressure contact having a shunt connection
with the source of current, a circuit controller in said shunt connection con-
sisting of two normally engaging contacts, an insulating strip operated by said
magnets to separate the contacts of said shunt circuit controller when the main
switch is closed, and a spark-arrester also operated by said magnets and adapted
to be thrust between the contacts of said switch when they are separated, sub-
stantially as and for the purposes set forth.
9. In an automatic circuit controller. for electric compressors the combination
of a movable contact having a fluid-pressure actuating connection, maximum
and minimum pressure contacts arranged to co-operate with said movable contact,
a switch in the compressor-motor circuit consisting of fixed insulated contacts
and of a number of spring contact arms carried by a rocking bar, two solenoid
magnets having a common core adapted by engagement with an arm on said
rocking har to close said switch, an incline carried by said core and adapted by
engagement with an arm of said rocking bar to open said switch, said magnets
being connected in series with each other, the coil of one magnet being connected
at one end with the ground or return and at the other end with said movable
contact, the maximum pressure contact being connected with the ground or
return and the minimum pressure contact having a shunt connection with the
source of current, a circuit controller in said shunt connection consisting of two
normally engaging springs or contacts, an insulatng strip carried by the core of
said magnets and adapted to separate the contacts of the shunt circuit controller
when said switch is closed and vice versa, and a spark-arrester also carried by
said core and arranged to be thrust between the contacts of said switch when they
are separated, substantially as and for the purposes set forth.
10. In an automatic controller for electric compressors the combination with
a movable contact having a fluid-pressure actuating connection, maximum and
minimum pressure contacts arranged to co-operate with said movable contact, a
main switch in the compressor-motor circuit, two magnets for operating said
switch, and a circuit controller operated by said magnets and arranged to be
opened when the main switch is closed and vice versa, said circuit controller being
arranged in a shunt connected with one of the aforesaid contacts and controlling
the current supply through said shunt to one of said magnets, substantially as
and for the purposes set forth.
11. In an automatic controller for electric compressors the combination with
a movable contact having a fluid-pressure actuating connection and maximum
and minimum pressure contacts adapted to co-operate with said movable contact,
of two magnets adapted to control the compressor-motor circuit, two switches or
circuit breakers operated by said magnets and controlling the circuits including
said magnets, each switch or circuit breaker being arranged to open when the
other closes and vice versa, said magnets having circuit connections so arranged
that when said movable contact engages the maximum pressure contact the com-

page_text: pressor will be stopped and when said movable contact engages the minimum
pressure contact the compressor will be started, substantially as and for the
purposes set forth.
12. In an automatic controller for electric compressors the combination with
a movable contact having a fluid-pressure actuating connection and maximum
and minimum pressure contacts adapted to co-operate with said movable contact,
of two magnets adapted to control the compressor-motor circuit, two switches or
circuit breakers operated by said magnets and controlling the circuits including
said magnets, each switch or circuit-breaker being arranged to open when the
other closes and vice versa, and insulating strips arranged to be thrust by said
magnets between the contacts of said switches or circuit-breakers when they
are separated, said magnets having circuit connections so arranged that when said
movable contact engages the maximum pressure contact the compressor will be
stopped and when it engages the minimum pressure contact the compressor will
be started, substantially as and for the purposes set forth.
13. In an automatic controller for electric compressors the combination with
a switch controlling the compressor-motor circuit and two magnets arranged to
open and close said switch, of a pressure gauge comprising a movable part
arranged to be actuated by fluid pressure and a contact connected with said
movable part by multiplying gearing, and insulated maximum and minimum
pressure contacts arranged on opposite sides and in the path of said movable con-
tact, said magnets having circuit connections with said pressure gauge so
arranged that when said movable contact engages the maximum pressure contact
the switch will be opened and the compressor stopped, and when said movable
contact engages the minimum pressure contact the switch will be closed and
the compressor started, substantially as and for the purposes set forth.
Dated this 26th day of February 1900.
BOULT, WADE & KILBURN,
Agents for the Applicant.