116 REPORT—1863.— 
It remains to be explained how electrical measurements can be practically 
made in electromagnetic units. Of all the magnitudes, currents are the most 
easily measured, provided the horizontal force (H) of the earth’s magnetism be 
known. Let a length (L) of wire be wound so as to form a circular coil of 
small section as compared with its radius (/). 
Let a short magnet be hung in the centre of the coil placed in the 
magnetic meridian, as in the ordinary tangent galvanometer, and let the 
deflection produced by the current C be called d, then it is easily* proved from 
the fundamental equation (5) that 
P) 
rae dan «dies Seem 
Thus, where the value of H is known, a tangent galyanometer only is required 
to determine the magnitude of a current in electromagnetic absolute mea- 
surement, although neither the resistance of the circuit nor the electromotive 
force producing the current may be known. The measurement of quantity 
ean be obtained from that of a current by a make-and-break apparatus, or 
«‘ Wippe,” in a well-known manner, or by measuring the swing of a galvano- 
meter needle when a single instantaneous discharge is allowed to pass through 
it (Appendix C. $25). If, therefore, we could measure resistance in abso- 
lute measure, the whole system of practical absolute measurement would be 
complete, since, when the current and resistance are known, equation (1) 
(Ohm’s law) directly gives the electromotive force producing the current. 
The object of the experiments of the Sub-Committee (made at King’s College, 
by the.kind permission of the Principal) was therefore to determine the re- 
sistance of a certain piece of wire in the absolute system, in order from this 
one careful determination to construct the material representative of the 
absolute unit with which all other resistances would be compared by well- 
known methods. 
There are several means by which the absolute resistance of a wire can 
be measured. Starting from equation (3), Professor Thomson, in 1851, deter- 
mined the absolute resistance of a wire by means of Dr. Joule’s experimental 
measurement of the heat developed in the wire by a current}; and by this 
method he obtained a result which agrees within about 5 per cent. with our 
latest experiments. This method is the simplest of all, so far as the mental 
conception is concerned, and is probably susceptible of very considerable 
accuracy. 
Indirect methods depending on the electromotive force induced in a wire 
moving across a magnetic field have, however, now been more accurately 
applied; but, before describing these methods, it will be necessary to point 
* The resultant electromagnetic force (f) exerted at the centre of the coil by a current (C) 
will, by equation (5), be f= 7p’ and the short magnet hung in the centre will experience a 
CLil 
we where 
ml=the product of the strength of one of the poles into the length of the magnet, or, in 
other words, its magnetic moment. The strength of the couple acting perpendicularly 
to the axis of the magnet, when it has deflected to an angle d under the influence of the 
: CLint ° : 
current, will be cos d = , at the same time the equal and opposite couple exerted 
on the magnet by the earth’s magnetism will be sin d Hm, hence 
out sind HE 
we cd LS 
t Phil. Mag. vol. ii. Ser. 4, 1851, p. 551. 
couple acting in a direction perpendicular to the plane of the coil equal to 
