IMPROVE- MENTS IN INSTRUMENTS FOR MEASURING ELECTRIC CURRENTS, RESISTANCES, AND FORCES
Year: 1873

page_text: LETTERS PATENT to John Toby Sprague, of Birmingham, in the
County of Warwick, Electrician, for the Invention of "IMPROVE-
MENTS IN INSTRUMENTS FOR MEASURING ELECTRIC CURRENTS, RESISTANCES,
AND FORCES."
'
Sealed the 20th October 1873, and dated the 29th April 1873.
PROVISIONAL SPECIFICATION left by the said John Toby Sprague
at the Office of the Commissioners of Patents, with his Petition,
on the 29th April 1873.
I, JOHN TOBY SPRAGUE, of Birmingham, in the County of Warwick,
5 Electrician, do hereby declare the nature of the said Invention for "IM-
PROVEMENTS IN INSTRUMENTS FOR MEASURING ELECTRIC CURRENTS, RESISTANCES,
AND FORCES," to be as follows :-
I arrange the wire of a galvanometer so as to form a series of circuits
having a definitely increasing influence upon a magnetic needle. I secure
the required multiplying ratio of the several circuits by balancing the
number of turns of wire in each circuit and its position and distance

page_text: from the magnetic needle against the length of the needle employed,
so that a number of turns in one position as regards the needle shall
have ten, one hundred, or any required number of times the deflective
influence upon the needle of the one turn or circuit which forms the basis
of the graduation of the dial, which therefore serves for all the circuits.
The dials of galvanometers are ordinarily graduated with degrees of
a circle but I graduate them to indicate definite units of electric
current, such as the Veber unit of the British Association, or any other
unit preferred; and for practical purposes to shew how many grains or
ounces or other measure of any metal the current is capable of
depositing, or to measure any actual work desired.
Hitherto galvanometers have been employed to measure electric
currents, and the resistances and forces connected with these currents
have been ascertained by various instruments and calculations, but I
make the dial of the galvanometer so that by employing one or more
Daniell's constant cells the resistance of the circuit may be read off in
Ohm's or other unit preferred; in like manner by making the resistance,
of the circuit one Ohm, or such multiple thereof as suits the instrument
or circuit used, the electro-motive force of any cell or battery may be
read off direct from the dial in Volts. For these several purposes I
either graduate the dial in concentric circles or use different quadrants
for the separate purposes, and I employ dials so graduated either with
the multiplying galvanometer before described or with any other form
of galvanometer in which a dial is employed.
In the case of instruments having a mirror and scale instead of a dial
I graduate the scale to fulfil the same purposes. I effect this
graduation in tangential galvanometers by ascertaining by experiment
the value of one deflection in any required unit, and then completing
the graduation by the law of the tangents of the angle of deviation; I
graduate sine galvanometers in the same manner. Having in this way
obtained a galvanometer, graduated as desired, I graduate any others
by placing them in the same circuit, with varying resistances and
battery powers, and making the graduation agree with that of the
standard galvanometer.
In some cases instead of graduating the dial itself, as described, I
graduate it in degrees as usual, but provide a table to accompany the

page_text: instrument, in which is shown the value, in each desired unit of the
several degrees of deflection, which table is constructed in the same
manner as is described for effecting the graduation of the dial itself.
on the 29th October 1873.
.
TO ALL TO WHOM THESE PRESENTS SHALL COME, I, JOHN
TOBY SPRAGUE, of Birmingham, in the County of Warwick, Electrician,
send greeting.
WHEREAS Her most Excellent Majesty Queen Victoria, by Her
Letters Patent, bearing date the Twenty-ninth day of April, in the year
of our Lord One thousand eight hundred and seventy-three, in the
thirty-sixth year of Her reign, did, for Herself, Her heirs and successors,
give and grant unto me, the said John Toby Sprague, Her special
licence that I, the said John Toby Sprague, my executors, admini-
strators, and assigns, or such others as I, the said John Toby Sprague,
my executors, administrators, and assigns, should at any time agree
with and no others, from time to time and at all times thereafter
during the term therein expressed, should and lawfully might make,
use, exercise, and vend, within the United Kingdom of Great Britain
and Ireland, the Channel Islands, and Isle of Man, an Invention
for "IMPROVEMENTS IN INSTRUMENTS FOR MEASURING ELECTRIC CURRENTS,
RESISTANCES, AND FORCES," upon the condition (amongst others) that I,
the said John Toby Sprague, my executors or administrators, by an
instrument in writing under my, or their, or one of their hands and
scals, should, particularly describe and ascertain the nature of the said
Invention, and in what manner the same was to be performed, and
cause the same to be filed in the Great Seal Patent Office within
six calendar months next and immediately after the date of the said
Letters Patent.

page_text: same is to be performed, to be particularly described and ascertained in
and by the following statement :-
In the first place I may state that my Invention mainly relates
to improved instruments such as galvanometers, which I construct
with several circuits having powers of deflection increasing in definite
ratios, as 1, 10, 100, so that the one graduation serves to measure
currents passing through them. The dials I graduate to indicate
currents in definite units, such as Vebers, or to measure any practical
work, such as the weight of metal deposited per hour. I also graduate
the dials to indicate directly the resistance of the circuit in ohms or
other units without requiring a resistance instrument by using a
standard battery, and to indicate the electro-motion force of a cell
or battery in Volts by making the resistance one or more Ohms as
required; and I can make tables to accompany galvanometers of any
kind to convert any arbitrary or indefinite scale of graduation into the
corresponding values in definite units.
Having thus set forth a general and cursory description of the nature
and object of my said Invention, I will now proceed to definitely
exemplify the same, and in further compliance with the aforesaid
condition in the said Letters Patent contained to particularly describe
and ascertain the nature of the said Invention, and in what manner
the same is to be performed or put in practice, wherein I shall refer
to the Drawings or Diagrams accompanying these presents, that is
to say :-
(Section 1.) Though the influence exerted by a conductor conveying
an electric current around a magnetic needle is usually considered to
be greatest when the conducting wire is placed vertically in the plane
of the needle's position of rest, yet my experiments shew me this is
not universally the case, for when the wire is near to the needle, as
in the usual flat coils used for galvanometers, I have found that the
deflection is increased as the wire is moved outwards to an extent
that is varied with the distance of the wire and the current passing.
In consequence of this an arrangement of two or more circuits having,
say, ten or one hundred fold the power of the weakest one in one
position of the needle may not preserve the same ratio of effect in
different conditions of current which has hitherto been of no con-
sequence because the deflections have not had a fixed value attached
to them.

page_text: (Section 2.) Now in order to obtain a multiplying ratio constant
or nearly so for all currents it is necessary to arrange the wires so
that the actions of different parts of the circuit balance each other;
this I effect by so forming the coils that each horizontal plane of
the circuit shall have an equal action upon the needle, and the form
which does this depends upon the relative lengths of the needle, and
of the coils, and upon the degree of deflection desired to be produced
for a fixed or unit current, the less the deflection desired the wider
must be the coils, or the shorter the needle in proportion to the length
of the coils. The proper form requires to be ascertained for any size
of needle, and I effect this by any means which enable me to measure
the effect of the several planes or layers of the coil.
The principle of the simplest mode is shewn in Fig. 1. A and B
are pieces of wood arranged to slide in a groove in a stand not shown
to such distance apart as suits the needle to be tested, and which is
hung between them in such a manner that A and B represent the
space between the two coils of which a galvanometer is composed.
1, 2, and 3 are either grooves or rows of holes in which studs can be
arranged; +and are metallic grooves with connections to a battery.
The studs are of metal and so arranged as to represent the corners
of the rectangles (or circles if desired) of the several layers of the
wire, and so that by winding backward and forward upon them as
closely as possible a flexible wire or metal cord they may actually
constitute such a layer, the width of which may be varied until the
layer becomes a single flat conducting circuit of such width as to
produce, with a given or unit current, the required deflection. The
studs are then altered so as to form a succession of such rectangles,
producing the same deflection with the same current, and the widths
must be such that each layer produces two such deflections aliko
for each layer, one with a small current and one with a large current,
to obtain which result it will be necessary to extend the coils to a
width at least equal to half the length of the needle. The width
of each layer thus formed is to be marked on a scale representing
the cross section of the instrument, and the several planes thus marked
will form the outline of the coils of wire to form the galvanometer.
As an illustration I give in Figure 2 the outline obtained for a two
inch needle, and a deflection of 30° for a current of one Veber.
From the centre A, with radius two inches, an arc is struck from C,

page_text: and a point G, at 15° is taken; from G, with radius G, B, an arc
B, H, is struck which is the outline of the outer edge of the wires
of the galvanometer; the Figure now shows à section of a half of one
side of the coil frames; A, F, is the line of suspension of the needle;
A, B, its plane of motion; E, J, and B, D, the upright frame of the
coil and the shaded part, any convenient filling up leaving H, E, J,
as the space in which the wires are to be wound. The properties
of this space are such that each horizontal layer will have equal
action upon the needle if filled with a flat conductor, or as many
times such action as it contains turns of wire completely occupying
it. Thus, if nine hundred turns of wire are wound upon the inner
portion, and an end brought out for a conductor, then continued for
ninety turns, again for nine finally ended by one flat riband of copper,
then if the graduation is based upon the action of this single turn the
several circuits will with the same current excrt 10, 100, and 1000 times
the influence of the first, and require the readings of the dial to be divided
by those figures for current or multiplied by them for resistance.
Section 3. It is of great importance to arrange all the connections so
that no influence shall be exerted upon the needle unless by the coils
themselves, especially when a circuit of a single turn forms the basis of
the multiplying power and of the graduation. For this purpose, I
construct a commutator composed of a series of metallic blocks, one for
the end of each separate or successive circuit, and a single collecting
block facing and parallel to these, making connection as usual by a plug
or spring between this collecting block and the block completing the
desired circuit. The terminal binding screws are connected one to the
collecting block, the other to the beginning of the circuits of the
coils, but instead of making these connections as usual by a single wire
I employ two wires from each terminal, and lead them so that wherever
a wire is led from a terminal or conveying a current in one direction,
there shall be in its immediate vicinity and neutralizing its effect upon
the needle a wire carrying a current in the opposite direction. The best
position for this commutator and for the terminals is upon the base
board of the instrument, and upon a line with the space between the two
halves of the coils, but any position may be used if the connections are
arranged as described, to neutralize all action upon the needle in any
position it may occupy.

page_text: electro-metallurgy on a practical scale, instead of placing the needle
within the coil I mount it outside a single turn, so as to obtain the
action due to the difference between those of the upper and lower parts,
and if necessary cause a part of the current to traverse a second circuit,
weakening the effect of the other by being laid in a reverse direction,
or I mount the dial and needle upon a slide so as to alter its distance
from the wires, or I make the conductor itself variable, as shewn in
section in Figure 3. + and are plates of metal fixed upon a stand and
connected to a battery; A is a sliding contact which completes the
conductor, and by its distance varies the resultant action on the needle.
Or, I mount the needle upon the stand and surround it with a single
turn of wire, as in a tangent galvanometer, but I make this turn
moveable on an axis in the line of the needle's position of rest, causing
the current to enter and leave it by means of two springs which form
the axis at one end of the frame. As the conductor is inclined from
the vertical line its effect is diminished till it becomes nothing when
horizontal.
In all these forms I attach scales to the moveable portion to indicate
the value each position gives to the readings of the dial.
Section 5. In the forms hitherto described, when once adjusted the
deflections will be the same for the same current, whether the magnetism
of the needle be strong or weak, except so far as the friction or resistance
of the support may affect it. This will not be the case when astatic
needles are used, or when the instruments are to be employed in
circumstances where the directive influence of the earth will be
neutralized. In such cases I employ an external controlling magnet. to
supersede the earth's influence, and adjust it to suit any varying
influences by passing a unit current through the instrument by means
of a known force and resistance, and altering the governing magnet
until the corresponding unit deflection is obtained.
.
Section 6. Vertical galvanometers are not available for definite
measurements, owing to the varying relation between the magnetism
of the needles and the resistance to motion, but I provide them with
means of adjustment either by a controlling magnet, or by a shifting
weight acting upon the axis of the needles, and by means of a unit
current adjust this resistance as described in section 5.
Section 7. Hitherto galvanometers have been generally graduated in
mere degrees of arc or in arbitrary divisions, so that the observations of

page_text: different instruments had no relation to each other, and were nearly
useless to other people unless accompanied by several details involving
calculations, but I graduate galvanometers to show the actions of the
current in different units, rendering all instruments and observations
comparable and universally available. I effect this graduation in
tangential galvanometers by maintaining a steady deflection for a
measured time, causing the current to do some chemical work at the
same time, such as depositing copper. I measure this also and reduce
the observed results into the terms of one chemical equivalent in grains
per ten hours, copper being 31.75 grains in 10 hours. This unit of:
current I call a ". chemic," and use as the base of a system of measure-
ment. A current of 5.68 chemics is a current of one Veber per second,
and this gives me the unit of the British Association system of measure-
ment. By the known laws of the instrument I calculate from the
observed deflection that due to the unit preferred, and then graduate the
dial to that unit by the well-known laws. I graduate Sine galvanometers
in a similar manner.
Section 8. Having thus obtained a galvanometer exactly graduated, I
graduate any others by placing them with it either in a series or single
circuit, or else in multiple arc or derived circuits, with such resistances
as may be necessary to divide the currents as desired between the
instruments. I then by varying the current by well-known means
produce upon the standard instrument a succession of deflections, and
mark the corresponding indications in the dial to be graduated.
Section 9. With a proper set of resistance coils and knowing the force
and resistance of the cells employed, any galvanometer may be
graduated directly by means of Ohm's laws, by calculating the con-
ditions necessary to produce any required current or unit deflection
and then altering the total resistance so as to multiply or divide this
unit and marking on the dial the deflections produced. A dial obtained
by any of these modes is applicable to any galvanometer of the same
construction but when great accuracy is required each instrument
should be graduated by itself.
Section 10. Hitherto electric resistances have been measured by means
of coils, &c., and by various calculations but I graduate galvanometers to
shew the resistance of the circuit directly by using a standard cell
of known electro-motive force. At present the only cell practically

page_text: avanable as being constant and capable of unnorm reproduction iS TO
Daniell, and I employ one composed of copper in saturated solution of
sulphate of copper and zinc in a half saturated solution of sulphate of
zinc, and I assume according to the best authorities that the electro-
motive force of this cell is 1.07 Volts. By Ohm's formula therefore
it is easy to ascertain the deflection due to 1 Ohm of total resistance on
a tangent or sine galvanometer which has been graduated for current as
described in Section 7 and to graduate this from this unit deflection
or knowing the resistance of the cell, and by making this up to 1 Ohm
and then varying the resistance to multiply or divide this deflection
any galvanometer can be graduated directly.
Section 11. To measure resistances it is only necessary to use with
the instrument a similar cell and read off the indications with any
additional resistance. I have for simplicity spoken of one cell and of
graduations from. single units, but the same principles apply in other
cases and only require the reading to be corrected accordingly. Current
is the same whether from 1 to 100 cells, but if a galvanometer
graduated to single units and for one cell shows one ohm, when ten
cells are used then the resistance is 10 ohms. Any cell may be used
with the instrument instead of the Daniell, in which case the necessary
correction would have to be made for the different electro motive force
multiplying by that of the cell used and dividing by 1.07; in this way
the internal resistance of any cell may be measured.
Section 12. I indicate electro-motive force by graduating upon the
Veber unit of current and making up the resistance including that
of the cell or battery to one ohm or such multiple thereof as is required
by the power of the instrument and the number of cells used and the
reading in Vebers is the electro-motive force in Volts when corrected as
required by the multiplying actions of the instrument and the resistance
used.
Section 13. I prefer to use separate quadrants of the dials for the several
units, and to divide or multiply these by the means described in sections
2 and 4, or by shunts as is commonly done when the current is to be
passed for a short time only; but when there are several circuits in
one instrument and their ratios are not sufficiently definite then I use
different quadrants for the different circuits and effect the different
graduations in concentric arcs.

page_text: Section 14. For use in large practical operations I graduate such
galvanometers as are described in section 5 or any desired form of
galvanometer to show the actual work the current is capable of doing
in such measures as the number of ounces of metal deposited per hour..
Such graduation I effect by calculating the value of the required
unit, in Chemics or Vebers of current, and making the corresponding
deflection the unit of graduation of the dial.
Section 15. In the case of reflecting galvanometers I graduate the
scales upon the same principles as described for dials, arranging the
scale at such distance as to produce a unit deflection by the proper
unit conditions of current.
Section 16. It is possible to substitute for the graduations described
the ordinary graduation into degrees of arc or any arbitrary scale, and
to place as an adjunct to the instrument a table showing the value of
these divisions in any of the various units of current, resistance, and
clectro-motive force, such values being first ascertained for that instru-
ment in the manner I describe for graduating dials, but this would for
most purposes be much less convenient than graduating the dials
themselves.
Having now particularly described and ascertained the nature and
object of my Invention and in what manner the same is to be performed
or put in practice, I hereby declare that I claim the Invention of
improvements in instruments for measuring electric currents, resistances,
and forces as herein-before set forth and described, wherein the following
are special points or features:-
1. The arranging of wires of galvanometers in the form described
in section 2 for securing several circuits of definite multiplying ratio in
one instrument.
2. The arrangement of commutator and also of connections described
in section 3 when used with a galvanometer with several circuits for
purpose of neutralizing all actions except those of the coils.
the
3. The various forms of galvanometer described in sections 4 and 6
when combined with dials shewing current in definite units and
particularly as described in section 14.
4. The application to galvanometers of any .construction of dials
graduated to show the resistance of the circuit without the use of
measured resistances.
w

page_text: 5. The construction of galvanometers substantially as described, and
provided with dials showing current resistance and electro-motive
force in definite units and the use of scales so graduated.
6. The combination of tables with galvanometers of any kind so
as to convert any arbitrary or indefinite scale of graduation into the
corresponding values in any definite units.
In witness whereof, I, the said John Toby Sprague, have here-
unto set my hand and seal, this Twenty-seventh day of October,
in the year of our Lord One thousand eight hundred and
seventy-three.
JOHN T. SPRAGUE. (L.S.)
Witness,
T. MORGAN,
Secretary,
Inventors' Patent Right Association, Limited,
21, Cockspur Street,
S. W.