12 
ME. LATIMER CLARK ON A STANDARD VOLTAIC BATTERY. 
on a pair of theodolite plates ; and thus readings could be taken to half minutes. The 
experiments were performed within five miles of the Eoyal Observatory. The value of 
H, a knowledge of which is necessary for the determinations with this instrument, was 
kindly supplied to me by the Astronomer Royal for each day on which the observations 
were taken. No iron was near the instrument. 
A difference of potential equal to that of one standard cell was maintained between 
the poles of the sine galvanometer, by the use of an auxiliary battery, rheostat, and 
galvanometer, in the manner described when treating of the dynamometer observations. 
The following Table gives the results of these experiments. 
Date. 
Value of H. 
Value of E. 
Remarks. 
9 Feb. 
1*788 
1-45605 
> 
1 
9 „ 
„ 
1-45457 
9 „ 
,, 
1-45400 
(Galvanometer wound with 8 turns 
10 „ 
1-788 
1-45809 
[ German silver wire. 
10 „ 
„ 
1-45669 
11 „ 
1-788 
1-45799 
1 
18 „ 
1-787 
1-45566 
i 
Rewound with 28 turns German 
19 „ 
„ 
1-45671 
J 
silver wire. 
19 „ 
1-45680 
1 
I 
20 „ 
20 „ 
1-787 
1-45752 
1-45645 
1 
[Rewound with 27 turns German 
24 „ 
1-786 
1-45522 
1 silver wire. 
24 „ 
” 
1-45492 
J 
■ 
Mean 
value of E ... 
1-4562 volt. 
Temperature 15°-5 Cent. 
The observations are corrected for the temperature of the element and of the coils ; 
but the correction for the breadth and depth of the coil, according to Professor Clerk 
Maxwell’s formula*, was so small as only to appear in the fifth place of decimals, and 
was therefore neglected. The instrument was rewound twice with various lengths of 
wire. 
We have therefore the mean value of the electromotive force of the standard cell,— 
volt. 
1. As determined by the electrodynamometer (18 observations) 1-45735 
2. As determined by the sine galvanometer (13 observations) . 1-45621 
Mean value of E .... 1-45678 
or, since no importance can be attached to the figures beyond the third place of decimals, 
1*457 volt, equal to 145700 absolute electromagnetic units. 
The uses of this standard element to practical electricians are sufficiently obvious. It 
may be used for determining the electromotive force of other elements by the use of an 
electrometer or by the discharge from a condenser. Or a condenser having a capacity 
of 1-457 f arad charged by the standard cell would contain the B. A. unit quantity of 
electricity (one Weber), or pjLy of the absolute unit of quantity. 
* British-Association Report, 1863, p. 170. 
