MR. J. E. H. GORDON ON MEASUREMENTS OF ELECTRICAL CONSTANTS. 421 
figs. 2 and 3). It is moved backwards and forwards by a screw B, pressed by a spring 
against a hardened steel plate at C (fig. 3). The screw inside D turns in two collars, 
one fixed to D and the other only kept from revolving, and forced away from the first 
by a stout spiral spring. This prevents what is called “back lash,” i.e., it ensures that 
the longitudinal motion shall be reversed at the same time as the motion of the screw. 
Fig. 2. 
Fig. 3. 
A scale divided to - 5 - 0 inch is engraved upon D, and a vernier fixed to one of the 
uprights reads the position of the plate to x'o&o i^ch. The scale is read by a micros¬ 
cope, fixed some three inches distant on the case of the instrument. 
The five plates are enclosed in a glass case, like a balance case, about 15 inches long. 
It passes below the stages, and holes in the top glass admit the four steel rods. A 
