ME. LATIMEE CLAEK ON A STANDARD VOLTAIC BATTEEY. 
13 
It is also of great value for maintaining a current of known strength in any circuit for 
the purposes of experimental research. Thus if it be desired to produce in any circuit 
(a b, fig. 5) a current equal to the B. A. unit of current (tuo absolute units), it is only 
Fig. 5. 
.4 -57 OHMS. 
necessary to insert in the circuit a wire R having a resistance of 1-457 ohm, and to 
connect to each end of this wire the poles of a standard cell, c, with a galvanometer, g, 
and to vary the strength of the current in a b until no deflection is produced on the gal- 
vanometer ; the current through a b will then be equal to one B. A. unit of current, or 
1 farad per second, whatever its length or resistance. 
By varying the resistance of R, or by varying the number of elements c , any given 
current can be steadily maintained through a b at pleasure ; on the other hand, the value 
of any given current can be measured by so varying the resistance R that no deflection 
is produced on the galvanometer. The value of the passing current will then be 
C— 1 j/*' - farad per second. 
It is also evident that, knowing the value of E, we may determine the horizontal 
intensity of the earth’s magnetism, H, in any place quickly and simply by means of an 
ordinary sine or tangent galvanometer. 
Thus by transposing the equation (3) we have for the tangent galvanometer 
TT E 27 ra 
RKtan 
In fact the standard of electric potential is second only in importance to that of the 
standard of electric resistance; and the use of such a standard, combined with an auxiliary 
battery in the manner described in the foregoing paper, admits of a variety of applications 
which it is believed will be found of great value in electrical research. 
In conclusion I have to acknowledge the great assistance I have received from Mr. 
Herbert Taylor, C.E., and Dr. Alexander Muirhead, in conducting these experiments. 
