10 
ME. LATIMiEE CLAEK ON A STAND AED VOLTAIC BATTEEY. 
Each layer has 15 windings, and there are 15 layers, so that each coil has 225 
windings. 
The small coils were wound afresh by myself ; the brass channels for the reception of 
the wire were of different sizes, and the same number of turns could not therefore be 
wound on it. 
millimetres. 
Mean circumference of first coil 35 9 ’25 
Mean circumference of second coil 357 ‘45 
Mean depth of the coils 6 ‘67 
Mean breadth of the coils 10-52 
Mean distance apart 62-41 
Number of windings on first coil ..... 311 
Number of windings on second coil 327 
The moment of inertia of the suspended coil was determined from a great number of 
observations by different methods. 
(1) The coil was vibrated on a fine steel wire, and the moment was then increased by 
a gun-metal cylinder passing through the centre of the coil; the increased time of 
vibration was then observed, and the moment of the coil calculated by the formula 
T Z 2 d*\ t* 
(^ r i2'^16)/' 2 — f (1) 
where W is the weight of the cylinder in grammes, l and d its length and diameter in 
metres, and t' and t the times of vibration of cylinder and coil and of coil. 
(2) From the value so ascertained the dimensions of a gun-metal cylinder were 
calculated, having about the same moment of inertia as the coil when vibrating on a 
transverse diameter : two of these rings were accurately formed by Mr. Becker and 
carefully weighed ; their times of vibration were compared with that of the coil when 
suspended from the same wire. The corrected moment of the coil was then calculated 
from these times by the formula 
-w?( r ¥ : +®. 
( 2 ) 
where t and t' are the times of a vibration of the coil and the ring, r and r 1 the 
external and internal radii of the ring, 2 a its breadth, and W its weight. 
(3) The moment of inertia was also determined by the vibration about its longi- 
tudinal axis of a metal cylinder of small thickness compared with its radius, as suggested 
by Sir William Thomson, F.B..S. (Proc. Royal Society, vol. xiv. p. 294), the coil and 
cylinder being alternately vibrated on the same wire. 
The following were the results of the observations : — 
First system, mean of five observations .... 1*27641 
Second system, mean of twenty observations . . 1-27680 
Third system, mean of one observation .... 1*27795 
Moment of inertia employed in calculation . 
1-27691 metre-gramme. 
