422 MR, J. E. H. GORDON ON MEASUREMENTS OF ELECTRICAL CONSTANTS. 
hole in the side admits the rod D, so that a is inside the case, while the screw-head is 
outside. The dotted line in fig. 3 shows the position of the glass side of the case. 
The five plates are placed close to one end of the case (see fig. 5, Induction Balance). 
On the base, inside the case, are slides, in which stages for carrying the dielectrics 
move. The stages can be moved by rods projecting beyond the case. Thus a 
dielectric under examination can be placed between two of the plates, or removed from 
them, without opening the glass case. 
On the upper fixed stages are ebonite pillars with double-binding screws on them ; 
to the lower nut of each is attached a flexible spiral wire, leading to one of the plates 
by way of the steel rod. By means of the upper nuts the plates can be connected to 
other instruments. 
The mechanical slide. —One of the sliding stages—viz.: that used to place a 
dielectric between a and b* —has “ mechanical motions.” In addition to the rod by 
which it is drawn in and out, there are three other parallel rods, with milled heads. 
Turning, one gives a lateral motion—viz.: moves any dielectric placed on the slide 
nearer a or b. By turning the second the dielectric plate can be placed either exactly 
vertical or inclined a little in either direction; and turning the third gives the 
dielectric a small angular motion round a vertical axis. 
The Thomson electrometer is a quadrant of the simple form by Elliott ; one of 
White’s which I have was found to be unsuitable for use with the reversing gear. 
When, as in this case, the instrument is only used as an electroscope, the superior 
sensitiveness of Elliott’s pattern gives it great advantages. 
The callipers were a pair with especially long jaws, made for use in adjusting the 
plates of the balance. When laid upon a bracket fixed to the outside of the balance 
case, the jaws projected right in between the plates. This bracket could be inclined 
either up or down, for measuring at the upper or lower parts of the plates. The “ out¬ 
sides ” scale was used in the ordinary way for measuring the thickness of the dielectric 
plates. 
Determination of speed of wheel-break. 
The fly-wheel of the engine had five spokes. A. piece of card of this shape 
being held with its pointed end against the spokes, was struck five 
times in each revolution, and gave the note B; 
j:_ e 1 which corresponds to 495 
vibrations per second.! Dividing this by 5, we find that the wheel revolves 99 times 
* In practice, tlie dielectrics are only pnt between a and b. 
t ‘ Sound and Music,’ Sedley Taylor, p. 200. 
