Year: 1898

page_text: CHARLES WILLIAM SCOTT CRAWLEY, 16, Red Lion Street, Clerkenwell, E.C.,
Engineer, do hereby declare the nature of this invention to be as follows:-
My invention is for the purpose of rapidly determining resistances. It con-
sists of a galvanometer in series with a resistance, generally considerably larger
than that of the galvanometer, and a source of fairly constant E.M.F. Part of
the above resistance I make so that it can be easily varied, and I thus adjust
for any variations in the E.M.F. of the source. The resistance to be measured
I put as a shunt on the galvanometer. In this way I obtain greater deflections
for low resistances, proportionately to those for high ones, than if the galvano-
meter were simply in series with the resistance to be measured, and thus obtain
a more useful scale while at the same time using an ordinary source of E.M.F.
such as one or more ordinary cells, considerable resistances in series with these,
and consequently a small total current. I sometimes instead of putting the
resistance to be measured as a variable shunt on the galvanometer use a fixed
shunt, and put the resistance to be measured in series with the galvanometer.
Dated this 14th day of October 1898.
C. W. CRAWLEY.
I, CHARLES WILLIAM SCOTT CRAWLEY, of 16, Red Lion Street, Clerkenwell, in
the County of London, Electrical Engineer, 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, reference being
had to the accompanying drawings-
My invention is for the purpose of rapidly determining resistances. It con-
sists of a galvanometer, shown diagrammatically at a in Figure 2, in series with
a resistance b, generally considerably larger than that of the galvanometer'
and a source c of fairly constant E.M.F. Part of the above resistance may be
arranged as shown at 61 so that it can be easily varied, and I am thus able to
adjust for any variations in the E.M.F. of the source. The resistance d to be
measured I put as a shunt on the galvanometer, as shown.
The galvanometer may be of any well known type that shown in Figure 1
being
an ordinary permanent magnet moving-coil voltmeter with the resistances
in circuit arranged as shown diagrammatically in Figure 2, the terminals 1, 2, 3
and 4 corresponding in the two figures. The scale of the galvanometer is approxi-
rn

page_text: mately even, ie. the deflections are approximately proportional to the current
passing through it. The battery c is joined on to terminals 1 and 4, the plug e
inserted, and the various resistances such as in the instrument shown, 1, 2, 3 etc.
up to 20 are joined on to terminals 2 and 3, the scale is then marked with the
respective indications of the pointer corresponding to the various resistances;
the value of any unknown resistances joined on to 2 and 3 will then be indicated
direct on the dial. It will be noticed that the indications on the dial which
are approximately those obtained with an even scale" instrument as above
described are larger for the small resistances proportionately to the high ones.
Should the E.M.F. of the battery decrease, the resistance 61 can be lessened to
counteract the drop. This is most simply done by inserting the plug e without
any resistance between 2 and 3, which gives the deflection of the unshunted
galvanometer and the pointer should therefore read infinity; if it does not, it
can be adjusted by raising or lowering the variable resistance b¹.
*
I thus obtain a more useful scale, while at the same time using any ordinary
source of E.M.F. such as one or more ordinary cells, as at c, considerable resist-
ances in series with these, as at b, b', and consequently a small total current.
Instead of putting the resistance to be measured as a variable shunt on the
galvanometer, I sometimes use a fixed shunt s and put the resistance d to be
measured in series with the galvanometer, as illustrated in Figure 3, in which a
is the galvanometer, the resistance, c the source of E.M.F. and the variable
resistance, all corresponding to those shown in Figure 2.
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 method, in determining electrical resistances, of connecting the resist-
ance to be measured as a shunt on the galvanometer, the said galvanometer thus
shunted being in series with a resistance preferably large compared with that of
the galvanometer and preferably partly variable, for the purpose of obtaining a
more convenient scale, substantially as described.
2. The measurer for determining electrical resistances, substantially as
described and as illustrated in the accompanying drawings.
A. F. SPOONER,
323, High Holborn, London, Agent for the Applicant.