166 
H. J. Denham 
A Multiple Switch and Commutator 
for Thermocouples 
By H. J. DENHAM, M.A. 
I N the course of an experiment involving the use of several thermo¬ 
couples with the same galvanometer, the need for some form of 
multiple switch was severely felt. A simple mercury switchboard was 
tried, but condemned on account of the thermo-electric errors which 
it introduced, and it became apparent that the switch must be worked 
at constant temperature and shielded from the warmth of the hand 
and other influences; accordingly, the rotary commutator described 
below was designed and constructed by the writer to work in an oil 
bath at constant temperature. 
Description. The instrument (see figure) consists essentially of 
two parts: a base (^ 4 ) of ebonite, containing twelve or more mercury 
cups, connected in diametric pairs to the thermocouple or other cir¬ 
cuits, and two concentric grooves filled with mercury, connected to 
the galvanometer. The base is screwed at the centre to receive a 
metal stem long enough to reach well above the surface of the bath, 
and provided at the top with a removable pointer. On this stem 
works a sleeve of ebonite or glass, carrying at its upper end an en¬ 
graved actuating head (B) of xylonite, and at its lower end a selector 
disk (C). This disk (whose function is to prevent sediment from falling 
into the mercury) is provided with two rigid bridge-pieces of wire, 
which serve to connect the concentric grooves to the mercury cups, 
and is supported above the base by a short metal sleeve and washer 
(D). Connections to the mercury cups and grooves are made by holes 
drilled horizontally in the base, of such a diameter as to take the 
bared wire of the leads, each hole being provided with a clamping 
screw which holds the wire in place. 
Method of use. The switch should be installed in a bath provided 
with a stirrer and, if extreme accuracy be desired, a thermostat, the 
actuating head and pointer being allowed to stand well above the 
surface of the liquid. 
The actuating head is lifted to disengage the contacts from the 
cups, and rotated until the number of the circuit required is opposite 
the pointer, when it is lowered and contact is made. To reverse the 
polarity of the circuit, the actuating head is again lifted and rotated 
through i8o°, when the circuit number again comes before the pointer. 
To avoid confusion the two sets of numbers should be differently 
