DR, J. HOPKINS ON ON THE RESIDUAL CHARGE OE THE LEYDEN JAR. 491 
minutes ; it then decreased, and the potential was nil in half an hour ; the main positive 
return charge then came out, and was still rapidly increasing at 8.20, when the flask 
was again discharged. At 8.39 the same flask was charged negatively till 8.44, then 
discharged and charged positively for 45 seconds, insulated 15 seconds and discharged, 
and finally insulated at 8.45. Curve B (Plate 44) represents the subsequent potentials. 
It is seen that the return charge twice changes sign before it assumes its final character. 
The experiment was several times repeated with similar results. 
Sir William Thomson has informed the author, since these experiments were tried, 
that he himself performed similar experiments many years ago, and showed them as 
lecture illustrations in his Class in the University of Glasgow, but never otherwise 
published them, proving that the charges come out of the glass in the inverse order to 
that in which they go inf. 
4. When steel is placed in a magnetic field, mechanical agitation accelerates the 
rapidity with which its magnetic polarity is developed. Again, vibration reduces the 
magnetism of a magnet, or, so to speak, shakes its magnetism out. This would suggest, 
on the present hypothesis, that vibration would accelerate changes in the electric polarity 
of a dielectric, or shake down polarization and liberate residual charge. The following 
experiments verify this anticipation. The arrangement was as in (3). The flask was 
strongly charged for some hours, discharged at 4.45 p.m., and kept with the two coatings 
connected by a platinum wire, except for a few moments at a time, to ascertain the rate 
at which the polarization was decaying, till 9.48, when the flask was insulated 
and the number of seconds observed in which the potential rose, to 100, 200, 300, and 
500 divisions of the scale of the quadrant electrometer, every thing being as steady as 
possible. The flask was then discharged, again insulated at 10.18, and the develop- 
ment of the charge observed, the neck of the flask being sharply tapped during the 
whole time. The experiment was repeated quiet at 10.48, with tapping at 11.16. 
Column I. gives the time of beginning the observation, II., III., IV., and V. the number 
of seconds in which charges 100, 200, 300, 500 developed respectively. The periods of 
tapping are marked with an asterisk. 
I. 
II. 
III. 
IY. 
Y. 
9.48 
118 
240 
367 
624 
10.18 
80* 
140* 
185* 
320* 
10.48 
140 
285 
440 
750 
11.16 
120* 
210* 
310* 
540* 
The effect may appear small ; but it must be remembered that, the flask containing 
and being immersed in sulphuric acid to the shoulder, the vibration caused by tapping 
t These results are closely analogous to those obtained by Boltzmanst for torsion (Sitzungsberichte der k. 
Akad. der Wiss. zu Wien, Bd. lxx. Sitzung 8. Oct. 1874). From his formulae it follows that if a fibre of glass 
is twisted for a long time in one direction, for a shorter time in the opposite direction, and is then released, the 
set of the fibre will for a time follow the last twist, will decrease, and finally take the sign of the first twist. 
3 y 2 
