ON STANDARDS OF ELECTRICAL RESISTANCE. 159 
Personally being charged with the duty of preparing an historical summary 
of the various units proposed, I shall be grateful if you will favour me with 
any remarks as to your own labours in this field, or if you could oblige me 
with references to any papers or works in which the subject is treated. 
I am, Sir, 
Your obedient Servant, 
Fieemine JENKIN. 
Aprennix H.—Description of the Electrical Apparatus arranged by Mr. Flee- 
ming Jenkin for the production of exact copies of the Standard of Resistance. 
This apparatus is a simple modification of that generally known as “Wheat- 
stone’s bridge.” It contains, however, some special arrangements, in virtue 
of which various practical difficulties are avoided, so that very great accuracy 
can be ensured with comparative ease. The usual bridge-arrangement is 
shown in Plate I. fig. 9, where the irregular scrolls, A, C, R, 8, represent the 
four conductors of which the resistance is to be compared; the thick black lines 
show those portions of the circuit which join the coils with the four corners, 
U, V, Z, Y, and are supposed to have no sensible resistance in comparison 
with the coils; finally, the thin lines show connexions, the resistance of 
which in no way affects the accuracy of the comparison between the four 
coils. By this arrangement the four conductors, A, C, R, 8, are so connected 
with the galvanometer, G, and the battery, B, that no current passes through 
the galyanometer when the conductors bear such a relation to one another that 
the equation a =5 holds good; whereas a current in one or other direction 
A ge 8 
passes so soon as G is greater or less than RR: Thus the direction and 
strength of the current observed serve as guides by which the resistance of 
any one of the conductors may be gradually adjusted by shortening or 
lengthening the wire, until on the completion of the circuit no deflection 
whatever can be observed on the galvanometer, however delicate it may be, 
or however powerful the battery used. When this has been done, we may 
be sure that the above relation exists between the four conductors. In 
practice, it is seldom desirable to use powerful batteries; the test is made 
delicate by the use of an extremely sensitive astatic galyanometer. 
In speaking of the four conductors, A, C, R, 8S, which are generally all 
coils of wire of similar construction, although each fulfilling a distinct 
function, some difficulty often occurs in explaining readily which coil or 
conductor is referred to. They can of course be distinguished by letters, but 
this requires reference to a diagram on every occasion, and the writer has 
therefore been in the habit of distinguishing the four coils by names drawn 
from a very obvious analogy existing between this electrical arrangement 
and the common balance in which one weight is compared with another. 
The equality between the two weights on either side of a balance, when the 
index is at zero, depends on the equality of the arms of the balance; and if 
the arms are unequal, the weights required to bring the index to zero are 
proportional to the arms (inversely), Let A and C be called the arms of the 
electrical balance, while S and R are looked on as analogous to the standard 
weight and mass to be weighed respectively, and let the galvanometer needle 
* This statement holds good also if the battery and galyanometer wires, as shown 
in diagram, are interchanged. 
