257 
1909-10.] On an Electrically Controlled Thermostat. 
The source of electricity used was the Edinburgh Corporation lighting 
supply at 230 volts. 
A diagram of the connections is shown in fig. 2. 
The current for the relay and thermo-regulator is reduced by means of 
the two 8-candle-power glow-lamps a, a, which also prevent all risk from 
an accidental earthing of the wires. One of the chief difficulties encountered 
was the deterioration of the mercury contact due to sparking. This was 
overcome by arranging the mercury contact in parallel with the electro- 
magnets of the relay which controls the heating circuit. When contact is 
broken the current goes through the electromagnets, and the potential 
difference across the contact in b (fig. 1) is comparatively small. Sparking 
is thereby greatly reduced. 
When the thermostat is in continuous use it is necessary to clean the 
mercury contact about once a fortnight. To do this a small quantity of 
mercury is removed from the tube b by means of a long, narrow mercury 
pipette. The sides of the capillary are then rinsed once or twice with 
clear toluol. The mercury contact is finally covered with a little toluol. 
The whole operation requires less than five minutes. The contact in b is 
then re-adjusted. 
The relay is shown in figs. 2 and 3. 
The two electromagnets (figs. 2 and 3, c) are wound with 2620 turns 
of copper wire, 36 standard wire gauge (0T55 mm. diam.). The resistance 
of the electromagnets in series is 95'9 ohms. The diameter of the core is 
| inch. When contact is made in b (fig. 2) by the thermo-regulator, practi- 
cally no current goes through the electromagnets, and the rod g (figs. 2 
and 3) falls by its own weight, breaking contact at d and extinguishing the 
heating-lamps. The shorter arm of the rod bears a nut which acts as a 
counterpoise, and can be screwed out or in until an adjustment is arrived at 
which gives the greatest sensibility compatible with certainty of action. 
This arrangement is much less liable to get out of order than any spring 
