

an Electric Thermostat. All 
of endwise displacement. The hit-or-miss arm H has a 
comb K attached to it crosswise, the teeth of the comb being 
disposed downwards, and the throats between them serving 
to engage with the cam C, as occasion arises. The outer end 
of the hit-or-miss arm H carries an ebonite sector Hy Gigs 1). 
In considering the action of the cam we will confine our 
attention for a moment to one of its functions, that, namely, 
of causing the hit-or-miss arm H to rise and fall ugain for 
each revolution of the sleeve X. This action of the cam is of 
course due to its radius (reckoned from the axis of the 
shaft S) being variable (fig. 2). The sector of larger radius 
is called ae being uppermost at the instant when our 
observation commences, the hit-or-miss arm H is raised 
to its fullest extent, and the ebonite sector H, is lifted 
clear of the galvanometer-boom; after some seconds, the 
rotation of the sleeve X brings the sector C, uppermost, so 
that the hit-or-miss arm H is left free to fall until its descent 
is arrested. The extent of the fall permitted to the arm H 
depends upon the position of the galvanometer-boom G, at 
the time being. 
(a) If the oil immediately surrounding the copper arms 
of the bridge B is slightly too cold, the boom is deflected in 
the direction marked “cold” in the diagram, so that the 
ebonite sector H, clears the boom, and allows the arm H to 
come down as far as a stop-screw, thus attaining its eee 
position. At the same time the shunt-circuit brush J, 
virtue of its attachment to the same rocking-shaft R, as the 
arm H, has likewise reached its lowest position, in which it 
ean make contact with the drum D, as soon as the latter has 
turned a little further. The contact thus established causes 
a current to flow in the intermittent heating-coil until the 
cylindrical sector D has passed the shunt-circuit brush J, 
shortly after which the sector C, of the cam comes under the 
comb K, and once more lifts the arm H and the brush J to 
their highest position. 
(b) On the other hand, if the temperature of the oil 
surrounding the copper arms of the bridge B is slightly too 
high, the boom Gy, is deflected in the direction marked “ hot,” 
and as the cam C leaves the arm H free to descend, the 
ebonite sector H, is arrested upon the boom G,, so that the 
arm H and the shunt-circuit brush J do not reach so low a 
position as in the alternative case just considered ; it results 
trom this that no contact is established between the brush J 
and the drum D, no current being allowed to flow through 
the intermittent heating- coil. 
The sequence of operations just described would of itself be 
sufficient to ensure a fairly constant temperature in the 
