Improvements in Apparatus for Measuring Electric Currents.
Year: 1893

page_text: Date of Application, 10th Jan., 1893—Accepted. 18th Feb., 1893
COMPLETE SPECIFICATION.
[Communicated from abroad by EDWARD WESTON, of Newark, New Jersey,
United States of America, Electrician.]
Improvements in Apparatus for Measuring Electric Currents.
I, 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 consists in an apparatus for measuring alternating currents of
electricity and in certain improvements thereon whereby the instrument is rendered
independent of the effects of terrestrial magnetism and of temperature variations
and whereby the index may be caused to move over a proportionately increased scale
distance for each added increment force.
'
Referring first to Figs. 1 and 2, which represent the instrument in plan and
sectional view, the base, 1, is in the form of a box and contains two spools, 2, 3, of
insulating material, on which are wound the fixed coils, 4, 4*. Within the fixed coil:
are two movable coils, 6 and 7, preferably made of fine insulated wire, with the
several turns cemented together so that each coil becomes a solid ring. Coils 6, 7 are
wound in opposite directions, and are supported by the short connecting shaft 5 and
the end shafts 9. The end shafts are pivoted in step-bearings in the brackets 10, 11.
Spiral springs, 13 and 12, have their inner ends fastened respectively to shafts 9,
and their outer ends to arms, 15, 13, supported on brackets 11 and 10. The effect
of these spiral springs is to oppose and counterbalance the rotary motion of the
movable coils. Also supported on shaft 9 is an index-needle, 16, which extends
over the scale 17.
The arrangement of circuits, will depend entirely upon whether the instrument is
to be used as a voltmeter or as an ammeter. If as a voltmeter, the movable coils and
the fixed coils will be connected in direct series with the binding-posts 21 and 22.
If it is to be used as an ammeter, the fixed coils will be connected in multiple arc
relation with the movable coils between said binding-posts, and the relation and the
size of the conductor in the movable coils to that of the fixed coils will be such as
to insure the passage of a known percentage of the current through the movable
coils.
When the current to be measured passes through the coils of the instrument the
movable coils will set themselves in a new angular position, and their extent of
movement will be indicated by the index moving over the scale and will have a
relation to the difference of potential existing at the instrument terminals.
The employment of two coils greatly increases the sensitiveness of the instrument.
The use of the two coils wound in opposite directions is to render the instrument
astatic and to elimate the effects of terrestrial magnetism, which might have some
influence on the indications of a single-coil instrument unless the latter were properly
disposed with reference to the magnetic meridian. The position of the coils, and
hence the measurement of the current, is made by the movement of an index-arm, 16.
which is carried by the shaft 9. over the fixed scale 17, which is suitably marked
in electrical units.
Referring now to Fig. 3, there is here shown a device for maintaining constant
electrical resistance in a circuit, such, for example, as that of the instrument just
described, despite changes in temperature occurring in said circuit and due to the
thermometric variations in the surrounding atmosphere.
The construction of the fixed coils being substantially the same as hitherto set
forth, I apply to the exterior of the upper coil a thermometer bulb, 21, the tube of
which, 22* disposed on the scale-plate, shows the temperature of the coil. At p',

page_text: "
², p³, is a series of resistances, "over which moves the contact arm 26, and this device
is interposed in the wire 27 in circuit of the fixed coil. The resistances p³, p², p³,
are arranged between contact-plates, o, o¹, o², &c., which are marked to correspond
to the thermometer scale. When the arm 26 is placed on the contact plate o, then
the current passes through all the resistances p¹, p2, 23, and this may be conveniently
supposed to be the starting point of temperatures. If the heat of the coil increases,
. for example, so that the thermometer shows a temperature of 65 degrees, then, in
order to compensate for the increase of resistance in the coil, the arm 26 is moved
around to meet the plate o, marked 65 degrees. As each one of the resistances, p',
p², &c., corresponds to the increase of resistance in the coil due to the elevation
I of 5 degrees of temperature, it follows that by throwing out seven of these
resistances the increased resistance of the coil due to its augmented heat is
compensated for; hence it is simply necessary to note the indication of the
thermometer and place the arm 26 on the contact-plate marked correspondingly to
that indication and keep the resistance of the circuit in the instrument uniform.
The device so far described compensates for variations in resistance due to
In order to
changes of temperature produced by the heating effect of the current.
compensate for changes occurring in the circuit by reason of heat variations in the
surrounding atmosphere, there is interposed in the circuit of the fixed coil a body, 28,
of carbon or other material which decreases in resistance when increased in
temperature. This is to be proportioned so that for a given thermometric variation
in the atmosphere it will vary in resistance inversely to the change produced by the
same cause in the remainder of the instrument circuit. In practice a thin filament
of carbon connected directly in the instrument circuit is preferably employed.
The device which is illustrated in Fig. 4 is a means for causing augmented
deflections of the index of an electrical measuring instrument for equal increments
of moving force thereto applied; so that, instead of the needle moving over an
equal distance for each additional unit of current strength, for example, it will move
over a constantly increasing distance for each unit. In this way slight variations
in the current may become easily recognized.
In Fig. 4, 16 is the index-arm, as before, and to this is attached a second arm, 25,
which carries a weight, 29, and normally rests against a stop, 30, when the needle is
in zero position. A second stop, 31, prevents the needle from moving beyond the
position shown in dotted lines. When the arm 16 stands in the position indicated in
full lines in Fig. 4, then the weight 29 exerts the greatest leverage in opposition to
the movement of the needle, and this opposing force is of course referred to the
* needle pivot as a fulcrum. As the needle begins to move over its scale, the weight
describes the circular arc a band takes the position e d'ef, in which the leverage of
the arm decreases; more of its weight being thrown on the fulcrum and less acting
in opposition to the needle movement, until finally, when the arm 25 is in the
position shown in dotted lines, practically the whole weight rests on the fulcrum
and none of it opposes the motion of the needle. At this point, however, tlie
further movement of the needle is prevented by the stop 31. Mathematically the
resistance of the weight and leverage to the movement of the needle decreases with
the cosine of the angle assumed by the arm 25 in moving from its original position;
hence the needle will not meet a uniform resistance for each equal applied force
increment but a decrease of resistance, and therefore will travel over a greater
distance. Consequently, it will move over constantly longer scale divisions; so that
when it nears the end of its scale, very slight variations in the impelling force will
be represented by large movement of the index over the scale..
Having now particularly described and ascertained 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. An electrical measuring instrument constructed substantially as hereinbefore
described and containing two stationary coils, two coils wound in opposite directions
movable about a common axis and arranged in the fields of the stationary coils, a

page_text: spring opposing and counterbalancing the movement of the movable coils and an
indicating device for showing the extent of movement of said movable coils
2. An electrical measuring instrument containing an indicating thermometer and
a series of adjustable resistances whereby the electrical resistance in the instrument
may be maintained constant despite the changes of temperature occurring in said
circuit, substantially as described.
3. An electrical measuring instrument containing in circuit an electrical conductor
the electrical resistance of which decreases with augmented temperature, which
conductor is exposed to the atmosphere and so constructed as to render the resistance
in the circuit of the instrument substantially constant despite the influence of varying
atmospheric temperatures upon said circuit.
4. An electrical measuring instrument provided with means for opposing a
gradually decreasing mechanical resistance to the forward movement of the index-
needle whereby said index-needle is allowed to move over gradually increasing scale-
divisions for equal applied force increments.
HASELTINE, LAKE & Co.,
45, Southampton Buildings, London, W.C., Agents for the Applicant.