Improvements in and relating to Electrical Measuring Instruments.
Year: 1890

page_text: Date of Application, 29th Apr., 1890-Accepted, 28th June, 1890
COMPLETE SPECIFICATION.
[Communicated from abroad by MADISON MUNROE GARVER, of Newark, New Jersey,
United States of America, Electrician.]
Improvements in and relating to Electrical Measuring
Instruments.
I, FLENRY HARRIS LAKE, of the firm of Haseltine, Lake & Co., Patent Agents,
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 an instrument for measuring the fall of potential between
the terminals of a conductor in circuit, and consists more particularly first in means
of preventing a greater radiation of heat from or to one branch of a divided conductor
than from or to the other branch, and thus obviating differences in resistance in
said branches due to the heating of the material composing them, and second, in
means for adjusting the scale so as to cause a percentage variation in the relative
lengths of the scale divisions.
In the accompanying drawings, Fig. 1 is a plan view, Fig. 2 is a section on the
line X X of Fig. 1, Fig. 3 is a bottom view of the base board, Fig. 4 exhibits the
mode of winding the loop wires around the permanent magnet, Fig. 5 shows a portion
of said magnet with said wires wound and in place, Fig. 6 is a plan view illustrating
the mode and effect of displacement of the scale plate, and Figs. 7, 8 and 9 are
diagrams illustrating the mode of equalizing the temperature in the branches of
a divided conductor in circuit. Similar figures and letters of reference indicate
like parts.
1 is the base board. 2 is a permanent magnet secured by screws 36, Fig. 3.
4 and 5 are pole pieces having their opposing faces hollowed out to form a cylindrical
opening, and connected together by a plate 6 on one side, and plate 7 on the other,
by screws 8 and 9. They are also secured in contact with and between the arms of
the permanent magnet by screws 10. In the cylindrical space between the pole
pieces 4 and 5, is supported a cylinder 11 of magnetic material, by means of screws 12,
which pass through the plate 6. In the annular space between said cylinder and pole
pieces, is a frame of copper around which is wound the coil 13 of the instrument. The
frame is supported on pivot pins 14 in bearings supported in the caps 15. Connected
to each pivot pin, is a spiral spring 18. The outer ends of said springs are connected
to pivoted armis 16, 17, received upon sleeves of insulating material, so that a current
entering at one arm as 16, passes through the spring 18 connected thereto, and
thence through the upper pivot pin, thence through the coil 13, and thence through
the lower pivot pin, lower spring and arm 17. The current causes said coil to turn
on its pivot and to set itself at an angle, which angle is proportional to the strength
of the current passing through said coil. The movement of said coil is resisted by
the action of the springs 18.
From the plate 7 extends a horizontal projection 19 upon which is secured
the scale plate 20. In the scale plate 20 is made a curved opening, in rear of
which and attached by clips 21, is a mirror 22. 23 is a light disk carrying the index
needle 24.
25 is the back cover and 27 a metal cover which protects the working parts. 31 is
a cover fitting over the instrument, and secured by screws 37 in openings 38. In said
cover 31 are openings 32, 33, provided with glass.
Through projection 34 on plate 6, pass two adjusting screws, one shown at 35,
whereby the pole pieces 4 and 5 may be rocked on their pivots to level the scale
plate.
The cirenit in the instrument so far as described proceeds as follows: From the

page_text: binding post D to the bent rod C to wire, a, to arm 16, spring 18 and by the pivot
pin to coil 13, thence by the lower pivot pin to the lower spring 18, lower arm 17 to
wire b, to bent rod B, and thence to binding post A, or vice versa.
The foregoing construction and combination of parts are not of my invention, but
are the invention of Edward Weston.
The novel features in the instrument which constitute the present invention will
now be described. The bent rods B and C are of copper, and support metal plates E
and F. Connected at their respective ends to said plates, are a series of loops d, d¹, &c.,
of insulated wire, wound around the body of the magnet 2. In Fig. 4, the loop dis
shown extended and the loop d' is shown wound around the magnet. The wires a and b
are also connected respectively to the plates, E and F, and therefore, the coil 13 is in
one branch of the divided circuit, and the loops d, d¹, &c., in the other branch of said
circuit.
It will be obvious that the relative resistances of the two branches of the circuit
should be uniform, and it will be equally obvious that if one branch of the circuit
undergoes variation in temperature while the other branch does not, this uniform
relation cannot be maintained.
r
In Figs. 7, 8 and 9, which are illustrative diagrams, assume a circuit between
M and N divided into branches O and P. Near the conductor in branch O, Fig. 7,
let there be placed a mass of metal Q. Then assuming both branches O, P, to be
equally heated by the current, the branch O nearest the mass Q, will radiate its heat
to said mass, and therefore, will become cooled quicker than the branch P, and hence
the branch P being at a higher temperature than the branch O, will offer a greater
resistance for that reason to the current, than will branch O.
In Fig. 8, representing the same divided circuit, assume another mass of metal Q¹
placed in proximity to the branch P. If the two masses Q and Q be rigorously
equal then Qwill absorb as much heat from branch O as Q' will absorb from branch P;
but if the relation of the mass Q in all respects to the branch O he not rigorously
the same as the relation of the mass Q1 to the branch P, then the heat absorption by
one mass will not be the same as the heat absorption by the other, and again an
inequality in the temperature of the two branches may result.
Referring now to Fig. 9, still representing the same divided circuit, the masses Q
and Qare united to the bar Q of similar material, so that there may be a free con-
duction of heat from one to the other; and under this condition, it will be apparent
that equalization of temperature in both branches O and P must follow. The tem-
perature of the whole mass may be sufficiently elevated by one branch of the circuit
to prevent it absorbing heat from the other branch.
This principle is applied in the present instrument. The coil 13 is in close
proximity to the pole pieces 4 and 5 and the solid cylinder 11, and hence readily
radiates the heat due to the passage of the current through the coil, to these metallic
masses. The coil 13 is in one branch of the circuit from the plates E, F.. The
several loops d, d¹, d", &c., are in the other branch of the circuit also from said
plates E, F. By differentially winding the loops upon the magnet 2, of course any
magnetic effect upon the magnet enclosed, is prevented. The loops d, d, &c., radiate
their heat to the magnet; but the pole pieces 4 and 5 and cylinder 11 are also
united to the magnet, so that heat may be conducted throughout the whole as a unit
mass. Hence the conditions of Fig. 9 are reproduced. If the coil 13 is heated by
the passage of the current, the loops d, d', &c., will also be heated. The coil 13 in
one branch of the circuit, would radiate its beat to the cylinder 11 and pole
pieces 4 and 5, but this is prevented by the fact that these parts are heated by the
effect of the same current which passes through the other branch of the circuit
which includes the loops d, d¹, &c. Therefore the temperature of the coil,
and in fact, in both branches of the circuit in the instrument, is maintained prac-
tically uniform.
It
Referring now to the device for scale adjustment, this is best shown in Fig. 6.
sometimes happens that through slight errors in adjustment or construction, the needle
for a given current indicates on the scale a fraction more or less of a degree, for

page_text: example, than it should do. This, of course, may be corrected by alterations in the
coil, &c., but it is much simpler and easier to move the scale plate so that a percentage
variation more or less will be caused in various divisions of the scale. To illustrate,
and referring to Fig. 6, suppose that after moving from its zero position over to the
limit of its swing, the needle is found to be in error a distance of one degree; that
is, it may fail to reach the extremity of the scale, or pass beyond said extremity to
this extent. But it will be obvious that if the scale be moved bodily in such a direc-
tion that while the needle may still remain at the zero mark, the point on said scale
which the needle should indicate when at the limit of its swing, is brought into
proper position with reference to said needle, then the error may be corrected.
Upon the plate 19 is a projection G, which enters a slot H in the scale
plate 20. The slot H is slightly longer than the projection G, so that the plate 20
may be moved on the latter. The plate is also provided with two slots J, K, through
which pass the screws S uniting said plate to the plate 19. The sides of the projec-
tion G and of the slot H which receives it, and also the sides of the slots J, K, are
rigorously parallel with the radial line drawn from the pivot center of the
needle 24 to the zero mark on the scale. Hence, when the screws S are loosened,
the scale plate can be slid only in a line coinciding with the direction of the radius
aforesaid. Consequently, despite this movement of the plate, the needle still remains
pointing to the zero mark, as indicated by the dotted lines in Fig. 6. But, the effect
of moving the scale plate outwardly is to shorten the total scale, as shown by dotted
lines Fig. 6, while the effect of moving it inwardly is to lengthen said scale. If,
therefore, the needle with the scale plate in a certain position fails to reach the scale
limit by a certain division of the scale, one degree, for example, it is necessary simply
to slide the plate outwardly as shown in Fig. 6, until the needle moved to the end of
its path, indicates the exact limit; (in the Fig. the 50 mark); or, if it passes said
limit by the same amount, the scale is moved inwardly to accomplish the same
result. The consequence is, practically, a percentage variation throughout the
scale, (the slight eccentricity being immaterial); that is to say, if its length be
increased a certain distance at the 50 mark (to illustrate only), it will thus be
decreased half a degree at the 25 mark, quarter of a degree at the 12-5 point, and
nothing at the zero point.
Having now fully ascertained and described the nature of the said invention 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. The means for compensating for differences of temperature in the parts of a
divided circuit, in an electrical measuring instrument as hereinbefore specified.
2. The scale plate arranged and adjustable as set forth, so as to cause a percentage
variation in the relative lengths of the scale divisions.
Dated this 29th day of April 1890.
HASELTINE, LAKE & Co.,
45, Southampton Buildings, London, Agents for the Applicant.