ON STANDARDS OF ELECTRICAL RESISTANCE. 143 
25. Electromagnetic Measurement of Electrie Quantity.—A conducting body 
insulated at all points from the neighbouring conductors may in various ways 
be electrified, or made to hold a quantity of electricity. This quantity ($ 14) 
is perfectly definite in any given circumstances ; it cannot be augmented or 
diminished so long as the conductor is insulated, and is called the charge of 
the conductor. Its magnitude depends on the dimensions and shape and 
position of the insulated and the neighbouring conductors, on the insulating 
material, and finally on the electromotive force between the insulated and the 
neighbouring conductors, at the moment when the charge was produced. The 
well-known Leyden jar is an arrangement by which a considerable charge 
can be obtained on a small conductor with moderate electromotive force be- 
tween the inner and outer coatings which constitute respectively the ‘“‘insu- 
lated” and ‘‘ neighbouring” conductors referred to in general. We need not 
enter into the general laws determining the charge, since our object is only 
to show how it may be measured when already existing ; but it may be well 
to state that the quantity on the charged insulated conductor necessarily im- 
plies an equal and opposite quantity on the surrounding or neighbouring 
conductors. 
We have already defined the magnitude of a current of electricity as sim- 
ply proportional to the quantity of electricity conveyed in a given time, and 
we have shown a method of measuring currents consonant with this definition. 
The unit quantity will, therefore, be that conveyed by the unit current as 
above defined in the unit of time. Thus, if a unit current is allowed to flow 
for a unit of time in any wire connecting the two coatings of a Leyden phial, 
the quantity which one coating loses, or which the other gains, is the electro- 
magnetic unit quantity*. The measurement thus defined of the quantity in 
a given statical charge can be made by observing the swing of a galvanometer- 
needle produced by allowing the charge to pass through the coil of the galva- 
nometer in a time extremely short compared with that occupied by an oscil- 
lation of the needle. 
Let Q be the whole quantity of electricity in an instantaneous current, then 
A ese csr nh cer il hi 54 
Tv 
where C,=the strength of a current giving a unit deflection (45° on a tangent 
or 90° on a sine galyanometer), t= half the period or time of a complete 
oscillation of the needle of the galvanometer under the influence of terrestrial 
magnetism alone, and i= the angle to which the needle is observed to swing 
from a position of rest, when the discharge takes place; C, is a constant 
which need only be determined once for each instrument, provided the hori- 
zontal force of the earth’s magnetism remain unchanged. In the case of the 
tangent galvanometer, the formula for obtaining it has already been given. 
From equations (9) and (12) we have for a tangent galvanometer 
aE ye EOS MOY ots Si egy 
wn 
where, as before, = the radius of the coil, and n= the number of turns 
made by the wire round the coil. 
The quantity in a given charge which can be continually reproduced under 
fixed conditions may be measured by allowing a succession of discharges to* 
pass at regular and very short intervals through a galvanometer, so as to pro- 
* Weber calls this quantity two units—a fact which must not be lost sight of in com- 
paring his results with those of the Committee. 
