accompanies the discharge of an Electrical Conductor. 867 
that, in order to support the discharge already commenced, a 
much less considerable tension is required than is necessary to 
establish it in the first instance (Riess, Die Lehre der Reibungs- 
elektricitét, vol. 11. p. 636). When electricity once commences 
to pass in the disruptive form from A to C, an expansion is pro- 
duced in the intermediate layers of air, which amply suffices to 
explain the facility with which the same layers of air allow them- 
selves to be traversed by electricity of much lower tension. 
4. It is evident, however, that this expansion will depend on 
the quantity of electricity which in a given time passes from A 
to C. Consequently if the experiment be so conducted that, all 
other things being the same, a less quantity of the electric fluid 
passes from A to C, the expansion of the air will be less, and the 
discharge ought to cease sooner. But if the discharge ceases 
sooner, it will follow that the residwe will be greater. Now it is 
easy to yerify this theoretical conclusion. 
B is a metallic sphere of 0°31 metre diameter. This sphere is 
connected by means of metal wires, on one side with a sinus elec- 
trometer A (Pogg. Ann. vol. evi. p. 438), and on the other with 
an apparatus C, called by Riess an “ Entladungsapparat ” (Riess, 
Die Lehre der Reibungselektricitat, p. 365), and which consists 
of a moveable metal rod gh, turning at 2 on a horizontal axis 
provided with a ratchet and clapper, which can be moved by 
pulling the silk cord ad, so that the knob g can always be 
brought to a fixed distance from the metal ball f 
The ball f and the moveable rod g / are both supported on glass 
pillars, the former being united by means of a metal wire with 
one branch of the universal excitator D, of which the other 
branch communicates by similar means with one of the knobs 
of the spark-micrometer E. The other knob of the spark-micro- 
nieter is in connexion with the ground. Between the branches 
of the excitator D the substances are placed whose action on the 
residue we wish to determine. 
I confined myself to the comparison of the actions of brass 
wire and a cord of hemp steeped in water. Both these conduc- 
tors were 0°3 metre long, the diameter of the brass wire being :08 
millim., that of the hempen cord 2 millims. In each experiment 
the sphere B received the same charge measured by the electro- 
meter. The clapper which supported the rod gf was then 
pulled by means of the silk cord ad, the knob g descended 
