MESSRS. W. DE LA RUE AND H. W. MULLER ON THE 
instrument is a fixed holder with a screw and binding-nut like that for the upper 
terminal, this is in metallic communication with the clamp O', and holds in the 
above figure a terminal in the form of a disc D. To the top of the screw is fixed 
the ebonite wheel A, to which is fastened a metallic ring divided into 20 parts, 
each representing ywfroth of an inch ; as the divisions are 0 - 35 inch apart at the 
outer periphery @f the disc, it will be obvious that the toAoo ma y approximately 
estimated without difficulty. On the top of the right-hand pillar is a vertical scale 
by which the number of whole revolutions of the screw is read off. Before com- 
Fig. 1. 
mencing an experiment the discharger is connected with a battery of 10 cells 
through a detector-galvanometer, and the terminals approached cautiously until the 
motion of the galvanometer indicates contact; the length of the spark of 10 cells 
is so small that the reading of the micrometer is taken as zero. The screw is 
run up for a greater distance than the anticipated length of the spark to be measured, 
and when the micrometer has been connected with the battery it is gradually 
approached by steps. 
The connexion between the micrometer-discharger and the battery is effected by 
means of the discharging key, shown in the figures 2 and 3, which we have designed 
specially for our battery, as the ordinary form of doubly reversing key, even when 
made much larger than usual, was found not to answer for the high potentials we 
employ, in consequence of the voltaic arc continuing the current after metallic contact 
had been broken. 
The battery is connected to the discharging key at the insulated standards in 
