420 MR. J. E. H. GORDON ON MEASUREMENTS OF ELECTRICAL CONSTANTS. 
Description of instruments [continued). 
Die 5-plate induction balance.-— This instrument was invented some years ago by 
Sir William Thomson. 
Only a rough lecture-model was constructed at the time of the invention. The pre¬ 
sent instrument is the first which has been made for purposes of measurement. 
The working drawings were made by me from sketches and verbal explanations sup¬ 
plied by Professor Maxwell. 
The instrument was constructed with great skill by Mr. Kieser, of the firm of 
Elliott Brothers. 
Fig. 1. 
It consists of three circular metal plates, a c e, each 6 inches diameter, and two, b cl, 
each 4 inches diameter, arranged as shown in fig. 1. There is a space of rather 
more than an inch between each plate and the one next it. The plate a is movable 
parallel to itself, so that it can be placed either in contact with b or nearly three mclies 
from it. The other four plates are fixed. The plates bccle are supported from 
above by steel rods. The lower end of each rod is screwed into the upper edge of a 
plate, the upper into an ebonite plug fixed into a small triangular horizontal brass 
plate, at the corners of which are levelling screws. The screws rest on a flat brass 
stage, a slit in which allows the rod to pass through. As there is not room for all 
foui’ triangles close together there are two stages one above the other. 
The plates c e hang from the lower stage, and b cl, which are furnished with longer 
rods, from the upper one. The triangles when adjusted are clamped by screws which 
for c e are fixed in the second stage, while for b cl the clamps are carried on a third 
stage made especially to hold them. Four stout brass pillars support the three stages. 
The feet of these pillars are screwed into a large brass plate let into the wooden base 
of the instrument. On this plate also stand the pillars carrying plate a. Plate a is 
fixed to the end of a brass rod of section 
from which it is insulated by a block 
of 
ebonite (E, fig. 3). This slides on two pillars (A A, figs. 2 and 3), which it touches 
only on its inclined surfaces (fig. 2). It is pressed downward by stout springs (a a, 
