OF ELECTROSTATIC UNITS IN THE ELECTROMAGNETIC UNIT. 
415 
V'= difference of potentials between the disk and the opposed plate connected 
with the other pole of the battery. 
D=distance through which the opposed plate has to be moved from its zero 
position to bring the disk again into the sighted position after one of the 
poles of the battery has been connected with the opposed plate. 
D'= distance through which the opposed plate has to be moved from its zero 
position to bring the disk again into the sighted position after the other 
pole of the battery has been connected with the opposed plate. 
Then V =D 
V f =Dy/ 
8ttF 
■X* 
V— V f D— D f 
/SttF 
a/ -^-= difference of potentials between the two poles of the battery. 
It was not necessary to obtain D and D' separately, D — D' being given in the difference 
between the two electrometer readings got by connecting first one pole of the battery 
and then the other with the lower plate of the electrometer. 
The mechanical displacement due to F was determined by a long series of experiments, 
in which a known weight was distributed over the disk and the displacement of the disk 
observed. The average result of the later weighings was that 200-55 micrometer- 
divisions represented the displacement due to a force of *6 gramme weight. A similar 
series of observations in the Laboratory of the Old College gave 200-6 as the average 
displacement. In the comparisons then made 200-6 was employed, but the difference 
being only that of ^ per cent, is inconsiderable. 
II. Electrodynamometer. 
Let n, n'= number of turns on the two fixed coils respectively. 
r, r'=mean radii of the fixed coils. 
b = distance of the mean plane of the fixed coils from the centre of the movable 
coil, or half the distance of the mean planes from one another. 
C= strength of current in the coils. 
2 rnr and h m'r 1 are the effective lengths of wire in the coils. 
Let a and a ! = angles subtended at the centre of the movable coil by the radii of the 
fixed coils. 
Then the effective component of the magnetic force at the centre of the movable coil 
due to the fixed coils is 
2irnrC . Virn'r'C 
^+F Sm “+ 
sin a 
\ (r 2 + 6 2 ) 2 
27 mV ,a 1 
( r /2 + 6 2 ) f )- 
For each of the coils the following correction for breadth and depth of the coils given 
by Professor Clerk Maxwell* is to be employed. 
9 Electricity, Art. 700. 
