Year: 1886

page_text: Date of Application, 10th May, 1886.
Complete Left, 10th Feb., 1887.
Complete Accepted, 10th May, 1887.
[Communicated from abroad by JOHN W. HOWELL, of the City of New Brunswick,
in the County of Middlesex, and State of New Jersey, United States of
America, Electrical Engineer.]
1, BENJAMIN JOSEPH BARNARD MILLS, of the firm of Harris and Mills of 23
Southampton Buildings in the County of Middlesex Patent Agent, do hereby declare
the nature of said invention to be as follows:-
This invention relates to certain improvements upon the electrical indicator set
forth in a prior provisional specification filed by me No. 2630 of 1886; the object being
to increase the convenience, economy and accuracy of that instrument.
The indicator set forth in that patent is intended for use in electric lighting systems
to indicate variations in electro-motive force and it consists of a Wheatstone Bridge
having a high wire resistance and a lower wire resistance in one arm and an
incandescing electric lamp and a wire resistance in the other arm, the galvanometer
being connected between such arms and having a terminal adjustable along a resistance
coil situated between the high and low wire resistances in one arm of the bridge.
This apparatus is connected between the points of a circuit the electro-motive force
or difference of potential between which it is desired to measure or indicate, it being
adjusted for the standard electro-motive force.
An electro-motive force above this standard causes an increase in temperature and a
decrease in resistance of the lamp and causes current to flow through the galvano
meter by whose indications the difference of electro-motive force may be determined.
A decrease of electro-motive force causes a deflection of the galvanometer in the
opposite direction.
The above is as described in the prior provisional specification above referred to.
In the instrument described in that provisional specification and in all other
indicators in which incandescing electric lamps and wire coils have been employed,
difficulty has arisen from the fact that when the instrument remains connected with
the circuit, the wire coils continually increase in temperature from the current passing

page_text: through them (although the electro-motive force may remain constant) and thus
increase in resistance. The lamp however does not so increase.
Let a represent the high wire resistance and the lower wire resistance in one side
of the bridge, and the incandescent lamp resistance and c the wire resistance in the
other side:
C
Now when the bridge is balanced the product of a and c is equal to that of band 1.
But as the wire coils heat up, a, b, and è increase in resistance and thus both factors
of the product a c are increased and only one factor of the product b, and therefore
the bridge is thrown out of balance.
This is remedied by the present invention by so constructing or arranging the coil
above designated "" that it will increase in resistance proportionately more than a
and e and is therefore kept equal to a, c.
There are several different ways of accomplishing this. One is to divide the coil
which is placed in the same arm as the lamp, into two parts in multiple arc, or parallel
to each other. One part is a heavy coil which will carry most of the lamp current,
and the other a fine coil. These two coils are wound upon separate spools and upon
the fine coil spool is wound, over such coil the coil "b." Thus the fine coil under
coil helps to heat it and an additional effect is produced in the latter coil and the
changes of resistance are thus compensated for.
The compensating effect may be produced by simply winding the coil "" in a
very close coil without making any changes in the other coils. Such coil will then
heat more and increase more in resistance, proportionately than the others. Where
the greatest accuracy is not essential this method of compensation may be used alone.
The arrangement which it is preferred to use however is as follows. All the wire
coils, except that on which the galvanometer terminal is adjustable, are wound upon
the same spool so that they will all have and maintain practically the same tempera-
ture and the coil above designated as "b," or a part thereof, is made of a metal
different from that of the other coils and one whose resistance increases more under
increased temperature than does that of the other coils, as it is best to employ German
silver for the coils, a part of coil "6" is made of copper which material answers the
above named conditions.
Then as the resistance of the coils increases with the increased temperature that
of "b" increases more than the others and so again the change is compensated for.
A further advantage of winding all the coils on one spool is that it gives a greater
degree of compactness to the apparatus and hence allows the production of what has
never before been provided, viz.: an instrument of such small size that it can be
carried in the pocket, and by which the indications necessary in electric lighting
systems are readily made. For this instrument an incandescent electric lump of small
size is provided and all the parts are placed in a small flat metal box.
In the ordinary instrument of larger size described in the provisional specification
herein before referred to, the lamp has been placed upon the outside of the box which
contains the coils and galvanometer in such a position that it shines upon the scale at
the front of the box so that the indications can be observed.
Another feature of the present invention consists in supporting the lamp horizontally
above the box by attaching its socket to the base board making a portion of the top
and front of the box of glass and marking the scale on paper or other translucent
material, so that the light shines through the glass top upon the back of the scale and
the marks on the scale are readily seen through the glass front.
Dated M... 10: 1920
B. J. B. MILLS.
By Harris & Mills,
23, Southampton Buildings,
Agents.

page_text: This apparatus is connected between the points 1, 2 of a circuit whose electro-
motive force it is designed to measure or indicate, it being adjusted for the standard
electro-motive force.
An electro-motive force above this standard causes an increase in temperature and
a decrease in resistance of lamp and causes current to flow through the galvanometer
by whose indications the difference of electro-motive force may be determined. A
decrease in electro-motive force causes a deflection of the galvanometer in the opposite
direction.
The above is as described in the patent already referred to.
In the instruments constructed as described in that patent and in all other lamp
indicators difficulty has arisen from the fact that when the instrument remains con-
nected with the circuit the wire coils continually increase in temperature from the
current passing through them (although the electro-motive force may remain
constant) and thus increase in resistance. The lamp however does not so increase.
When the bridge is balanced the product of the resistances a and c is equal to that
of band. But as the wire coils heat up, a, b and c increase in resistance and thus both
factors of the product a c increase and only one of product bl, and therefore the
bridge is thrown out of balance.
The inventor remedies this by so constructing or arranging the coil & that it will
increase in resistance proportionately more than a and c and is therefore kept equal
to a c.
One way of accomplishing this is shown in figures 2 and 3. The resistance coils
are all wound as shown, upon suitable spools. Lamp l, coils b, e and galvanometer G
are all connected in the same manner as in figure 1. The remaining coil is however
divided into two multiple arc or parallel parts c¹ and e².
Coil is of heavy wire and is wound by itself upon one spool f: c², which is
preferably of the same resistance as coil c¹, is of finer wire and hence heats more and is
wound upon another spool f', and above coil c² is wound the coil b in a close coil as
seen in figure 3.
The effect of this is that the coil c² helps to heat coil b and therefore an additional
effect is produced in coil and the changes of resistance are thus compensated for.
The compensating effect may be produced by simply winding the coil in a very
close coil as seen in figure 3" without making any change whatever in the other
coils. Coil will then heat more and increase more in resistance, proportionately, than
the others.
Where the greatest accuracy is not essential this method of compensation may be
used alone.
The preferred arrangement for the purpose of compensation however, is that which
is shown in figure 5, and diagrammatically in figure 6.
The main feature here is the winding of all the coils-except coil e-upon the same
spool so that they will all have and maintain practically the same temperature and
making the coil or preferably only a part of said coil of a metal different from that
of the other coils and one whose resistance increases more under increased tempera-
ture than does that of the other coils. As it is best to employ German silver for the
coils, it is preferred for the purpose of compensation to make a part of coil b of copper
which answers the above nained conditions.
In describing this arrangement in detail the details of construction of the preferred
apparatus shown in figures 4, 5 and 6 will also be described.
A is a suitable box or case made principally of wood having for its back a base
board B, by which it may be attached to a wall or other support.
The box has a partition C upon which rests the galvanometer G, whose pointer D
extends out in front of a scale arranged as follows:-
Supports rising from partition C carry a brass frame k which holds the paper
strip made of a thin translucent paper on which the divisions of the scale are
marked. A portion E E of the top and front of the box is made of glass.
The lamp which is one of the resistances of the indicator is held in a socket F

page_text: supported upon base B and extends out horizontally above the top of the box. Its
light therefore shines through the glass top of box upon the back of the scale and
through the same so that the pointer and scale can be readily observed through the
glass at the front of the box, and can be seen more clearly and from a greater distance
than with the arrangements heretofore in use.
The wire coils a, b and c are all wound upon a frame e supported from the sides
of the box, one set of coils being wound upon another as shown and the whole
may be and preferably is in small compact instruments wrapped with a covering m
of asbestos paper or other non-heat conducting substance. All the coils are thus kept
at the same temperature. The coil e is wound separately upon a rod o and
upon one side has its insulation filed off to form a bare surface along which travels
spring d carried by sleeve upon rods and moved by screw taking into a projection
from the sleever passing through a slot in the front of the box, and which may carry
a pointer u moving across a scale u² above the slot to show position of the moving
contact. This construction is fully set forth in the application above referred to.
I is the device for compensating in the galvanometer for the earth's magnetism. It
is adjustable by a handle J outside the box.
In the bottom of the box is an aperture K covered with wire gauze L, and in
the sides of the box are other apertures M M. These assist in keeping the apparatus
cool.
The connections are clearly shown in Figure 6. Here the coils a, b, c, are, for
clearness of illustration, shown as wound separately upon the reel. The circuit con-
ductors are connected to binding posts 1, 2, on the base-board.
Four binding
posts 3, 4, 5, 6 are provided within the box. From post 3, which is connected directly
with 1, a wire 7 extends to lamp and wire 8 to coil a; from coil a connection is
through coil e and then through coil to wire 9 which goes to post 6, connected with
terminal 2..
l
The other terminal of lamp is connected by wire 10 with binding post 4 from
which wire 11 goes to coil e whose other extremity is connected with 9.
The galvanometer is connected between posts 4 and 5, and from post 5 a wire 12
extends to a screw which passes through the front of the box and sets against the
metal end piece w which supports rods o and s. This makes spring & the terminal of
the galvanometer circuit, movable upon coil e to balance the bridge.
A portion of coil is made of copper-all the rest of the coils being of German
silver. The object of this arrangement has been already fully explained.
A further great advantage of compensating for changes in resistance due to heat
and also specifically of winding all the coils upon a common spool or frame is the
additional compactness of the apparatus. This has enabled the inventor to make what
has never been produced before an instrument of such small size that it can be
carried in the pocket and by which the indications necessary in electric lighting
systems are readily made.
Referring to Figures 7, 8 and 9 N is a flat metal box having a lid or cover N¹, held
by a spring catch z. The coils a, b and c are all wound upon a common frame in
the same way as above described. All the metal conducting parts are supported upon
cushions of rubber O or other suitable insulating material. The galvanometer G is
placed in a compartiment at one end of the box covered with glass P. Its pointer w¹
plays above the scale y' which is placed upon a raised portion in the bottom of the
box. Coil e is upon a rod o and contact spring d travels upon the bared side of said
coil as hereinbefore explained. A key ¹ is provided to open and close the galvanometer
circuit.
The lamp is one of small size and its terminal wires are held upon contact plates t¹t¹
by spring clips s s¹.
This instrument is shown in full size in the drawing; it will be seen that it is
very small and compact, and at the same time equally effective with the larger
instrument.

page_text: Having now particularly described and ascertained the nature of my said invention
and in what manner the same is to be performed I declare that what I claim is :-
1. The methods or arrangements set forth for compensating for changes in
resistance of the wire coils due to increased heat in such coils, in indicators of the
character described.
2. The combination of the box or case, the scale within the same composed of
transparent or translucent material, the lamp outside the case and the glass panes
through which the light shines upon the back of the scale and through which said
scale is visible, substantially as hereinbefore described.