Improvements in Electrical Measuring Instruments.
Year: 1890

page_text: Date of Application, 11th Nov., 1890-Accepted, 13th Dec., 1890
[Communicated from abroad by EDWARD WESTON, of Newark, New Jersey, United
States of America, [Electrician.]
Improvements in Electrical Measuring Instruments.
HENRY HARRIS LAKE of the firm of Haseltine, Lake & Co., Patent Agents,
45 Southampton Buildings in the County of Middlesex 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:
-
The invention relates to improvements in an instrument for the measurement
of electricity, and more especially intended for use with alternating currents. The
instrument to which the improvements are here shown applied consists substantially
in a horizontal coil within which a vibrating coil is pivoted. To the pivots of the
vibrating coil are connected spiral springs, and the vibrating coil and fixed coil are
connected in circuit. When a current is established between the terminals of the
instrument, the vibrating coil assumes a position having a relation to the pressure
or strength of the current, and this position is shown by a needle connected to said
vibrating coil, which moves over a suitable scale. When the instrument is used to
indicate strength of current, this scale may be marked in amperes, or if pressure
of current is to be denoted, the scale is marked in volts.
The improvements applied to the aforesaid instrument, which are hereinafter set
forth, consist first, in a device for controlling the movement of the index needle so
that it will move slowly up to its indication and stay there without any violent
contact with its stops or undue oscillation; second, in a device for retarding or
dampening the motion of the index or pointer so that the said index instead of being
thrown rapidly to its marking and then caused to oscillate on the establishment of the
current, will move gradually to the point indicated and there come to rest; this device
operating through the medium of a friction brake; and third, a device for main-
taining constant electrical resistance in the circuit of the instrument despite changes
of temperature occurring in said circuit.
In the accompanying drawings, Figs. 1 to 8 inclusive exhibit the arrangement
of the adjustable stops for limiting the needle movement, Figs. 9, 10, 11, exhibit the
friction device for controlling the index, and Fig. 12 reprosents the temperature
regulating device. Similar letters of reference indicate like parts.
A is a fixed horizontal coil supported on the base hoard B. Within the coil is
arranged a vibrating coil A', (dotted lines, Fig. 2,) which is supported on pivots
received in arms C. Connected to the pivot pins of the coil at one end, are spiral
springs D and E. The outor ends of these springs are secured to pivoted arins F
and G. The upper pivot pin of the coil carries a light disk H to which is secured
an index needle I.
This instrument is to be used to measure current pressure. The circuit therein
proceeds from the binding post 1 by wire a to the arm F, thence to spring D, to
coil A', to lower spring E, arm G, and to binding post 2 by the wire b. The main
coil A is arranged in shunt circuit with the binding post 1, 2. This disposition of coils
is used when the main coil A is made of low resistance and the instrument is employed
for the measurement of current strength. When the instrument is to be used for
the measurement of current pressure, the vibratory coil A' and fixed coil A (the
latter being of fine wire) are connected in series with the terminals 1, 2. The
passage of the current between the terminals causes the movable coil A¹ to turn on its
pivots having a relation to the difference of potential between the terminals; and
this position is shown by the needle I on the suitably-marked scale J.
Referring now and more particularly to Figs. 1 to 8, Fig. 1 is a bottom view
of the instrument, Fig. 2 is a transverse section, Figs. 3 and 4 aro detail views,

page_text: Fig. 5 shows a portion of the guide rod provided with curved sectors, Fig. 6 is
a bottom view of the disk showing the slots or recesses therein, and Figs. 7 and 8
show different positions of the guide arm, disk and needle. On the lower pivot
of coil A' is a disk 3. Below the disk 3 and turning on a collar 4, which is received
upon the lower coil pivot, is an arm 5, and on each side of its pivot centre curved
pieces 6, 7, in which are formed concentric arc-shaped slots to receive pins 8, 9 which
project from the disk 3. This arrangement is shown in Fig. 3, but it may be
modified as shown in Figs. 4 and 6, in which curved slots, similar to 6 and 7,
are made at 10 and 11 in the disk 3; and the fixed pins which enter said slots
are carried by the arm 5. In the base board B of the instru:nent, there is a
curved slot 12 which receives the vertical pin 13 on the end of the arm 5; the said
pin having a handle which projects above the base board.
The operation of the device is as follows: referring more particularly to Figs. 7
and 8:
and
When the needle I is at rest, the pins 8 and 9 on the disk 3 will then be at the
extreme ends of the slots and 7 on the arm 5 (see dotted line at I). When
a current is established, the needle cannot fly over the whole scale, because the
pins 8 and 9 will meet the opposite ends of the slots in the arm 5; so that the
needle moves over only the distance to which the pins are limited by the curved
slots 6 and 7; and the position of the index or pointer will then be that shown
by full lines I, Fig. 7, after it is moved in the direction of the arrow.
But the
actual point to be indicated by the needle and due to the difference of potential
existing may be far in advance of the point at which the needle meets the limiting
stops. The arm 5 is then moved by hand in the direction of the arrow;
as long as the current tends to move the needle further along, the pins 8, 9 will
remain in position against the ends of the slots in said arm F. As soon, however,
as the needle has reached its proper indication, then it has no longer any tendency
to follow the moving arm 5, and this will be apparent by the needle becoming
left behind and showing a slight quivering as the arm 5 is carried further
forward. The position of the pins 8, is then shown in Figs. 1 and 8, and is
at some point along the length of the slots. The advantage of this arrangement
is that the needle can never have but a small range of motion caused at once
by the current, and hence it cannot acquire momemtum enough to be dashed
violently against far distant limiting stops, nor can it vibrate over any large area.
The arm 5 being moved by hand, the needle may be caused to move to its indi-
cation as slowly as desired, and without jerk or vibration.
Referring now to Figs. 9, 10, 11, on the lower pivot pin of the movable coil
is secured a disk 14 having a highly polished surface, against which bears a pad 15
of soft leather carried by the light metal arm 16. This arm is secured to the
leaf spring 17, which is supported at one end on the under side of the base board,
and is bent twice, as shown in Fig. 11. 17 is another leaf spring also fastened
at one end on the under side of the base board. The free end of the said spring 17ª
comes under a push button 18 of insulating material, which passes through a sleeve 19
on the base board.
The circuit in the instrument proceeds as follows:-
:-
From one binding post to the spring 17 and thence to the spring 17ª, when said
last-mentioned spring is pushed down by the pin 18, and so through the coils
to the opposite binding post.
The device operates as follows:-
When the pin 18 is pushed down, the spring 17ª is moved into contact with
the spring 17 and the circuit is closed through the instrument. The initial pressure
and consequent friction between the pad 15 and disk 14 is, however, main-
tained until the pin 18 is pushed down further; and then this pressure is slightly
relaxed so that the coil A¹ (dotted lines) is free to turn. But the motion of the
coil A is still opposed by the decreased frictional resistance of pad and disk; so
that the needle is caused to move slowly up to its reading. When that point is
reached, the pin being pushed down to its extreme limit, moves the pad 15 entirely

page_text: away from the disk 14, leaving the needle entirely free. The needle will then
remain at rest without oscillation as long as the current continue.
The device illustrated in Fig. 12 is intended to maintain the electrical resis-
tance in the circuit of the instrument constant despite changes of temperature in
the coils. If said coils are made of copper wire, the variations in resistance which
they offer due to changes in temperature, may be considerable. Thus, for example,
the resistance of copper increases at the rate of about two-tenths of 1 per cent.
for every degree Fahrenheit of temperature. Hence, an elevation of temperature of
but 5 degrees occurring in the coil will increase the resistance offered thereby to the
current by 1 per cent.; thus bringing a considerable source of error into the
instrument. In order that these variations in temperature may be recognized, a
thermometer 20 is provided having an elongated bulb passing in proximity to the
coil A, and secured in place by a strap 21. The thermometer tube is bent as
shown and arranged upon the scale plate J, and in a curve parallel to that of the
scale markings; so that the temperature of the coil is shown upon the same scale plate
as the regular indications of the instrument. A series of metallic contacts insulated
from one another are shown at 22 and marked o to o. Between the successive contact
plates are resistance coils p', p², p³, &c. Pivoted concentrically with the circular
contact plates is a switch arm 23, having contact springs 24 bearing upon the surface
of said contact plates. This series of resistances is connected in the circuit of the fixed
coil A and the contact plates o, o¹, &c. are marked to correspond to the thermometer
scale. Thus the contact plate os is marked 30 to correspond with the 30 degree
mark of the thermometer; the contact plate o corresponds to the 35 degree mark
of the thermometer; the contact plate o corresponds to the 40 degree mark, and
so on, the contact plate o corresponding to the 100 degree mark. When the
arm 23 is placed on the contact plate o, it will be evident that the current in the
instrument then passes through all the resistances p', p², &c. Let it be supposed
merely for purposes of illustration, that this is the starting point of temperatures.
Now suppose the heat of the coil augments so that the thermometer shows a tem-
perature of 65 degrees. This means an increase of resistance in the coil due to the
65 degrees difference of temperature. In order to compensate for this, the arm 23
is moved around until its contact spring 24 rests on the plate o'. Seven resistances,
p', p², &c., will thus be thrown out of the circuit; and as each resistance coil, pl, p²,
&c., corresponds to the increase of resistance in the coil due to an elevation of
5 degrees of temperature, it follows that by throwing out seven of these resistances,
compensation has been made for the increased resistance of the coil due to its
augmented heat. In practical operation, it is necessary simply to note the indication
of the thermometer and place the arm 23 on the contact plate marked correspondingly
to that indication in order to keep the resistance of the circuit in the instrument
uniform. It will be understood, of course, that the scale of temperature here given
is purely arbitrary, chosen merely for purposes of illustration, and that the number
of contact plates o, o¹, &c., here being one for 5 degrees of temperature, may be
increased or diminished in number as desired to correspond, for example, to evory
10 degrees change, or to one, or to even a fraction of degree change. The circular
contact plates may be conveniently arranged on the base board B.
Having now particularly described and ascertained the nature of the said inven-
tion and in what manner the same is to be performed, as communicated to me
by my foreign correspondent, I declare that what I claim is :-
1. In an electrical measuring instrument, an index controlling device embodying
a pivoted vibrating coil or other body caused by the current traversing the
instrument to assume a position depending upon difference of potential existing
between the instrument terminals, an index needle controlled by the coil, a plate
fast on the coil pivot and provided with pins 8, 9 and a pivoted rod having
recesses 6, 7 receiving said pins (or as an equivalent a plate having recesses 10, 11
and a pivoted rod having pins 8, 9): the said device operating and arranged sub-
stantially as set forth.

page_text: 2. In an electrical measuring instrument, a device for retarding and arresting
the movement of an index needle or other body actuated by the current traversing
the instrument and embodying a friction plate, a brake finger bearing thereon
and a circuit closer, the said circuit closer and brake finger operating successively,
substantially as hereinbefore described.
3. In an electrical measuring instrument, a temperature regulating device
embodying a means of indicating changes in temperature occurring in the instrument
circuit, a resistance and a means for varying the same comforinably to the indi-
cations of said heat indicating apparatus whereby the resistance of the circuit may
be maintained constant despite said temperature variations.
Dated this 11th day of November 1890.
HASELTINE, LAKE & Co.,
45. Southampton Buildings, London, Agents for the Applicant.