
an Eleectrie Thermostat. 413 
the higher level upon the boom Gy, the effect of the sector (, 
will be to raise the arm H clear of the boom Gy, leaving the 
latter free to take upits equilibrium position under the influence 
of such small current as may then be traversing the galvano- 
meter-coil, owing to want of balance of the controlling-bridge ; 
thus determining the greater or less descent of the arm H, 
after the sector C, has passed from under the comb K, as 
already explained. As regards the displacement of the 
eam ©, together with the drum D and slider W, along the 
axis of the shaft S ; it is apparent from the foregoing para- 
graph, that for every revolution of the spindle a displacement 
of 4 mm. to the left occurs, while a displacement of 8 mm. 
to the right occurs only for those revolutions in which the 
arm H misses the boom G,. Thus it follows that when the 
hit-or-miss arm H just hits the boom G, as often as it misses, 
the cam, drum and slider will only suffer small axial displace- 
ments on either side of a mean position, but on the whole 
will be without progressive axial motion. On the other 
hand, if the demand for heating current is greater than can 
be met by a steady average of one hit to one miss (that is to 
say, by one short-circuiting of the series-coils for two com- 
plete revolutions of the sleeve X), the galvanometer-boom G, 
will be more often deflected to the “cold” side than to the 
“hot” side, and the misses will preponderate over the hits, 
In consequence of this, the sector C, of the cam will operate 
more often than an average of once in every two revolutions 
of the sleeve X, and so the displacement of 8 mm. to the 
right will occur on the whole more than half as often as 
the displacement of 4 mm. to the left. The result will be a 
progressive displacement to the right, causing a wider part 
of the brass sector D to come under the brush J, so that each 
short-circuiting is of longer duration, and perhaps also causing 
the slider W to move from one of the blocks L to another, 
thus reducing the resistance interposed by the series-coils F 
in the main circuit. These effects will diminish the demand 
for such frequent short-circuiting of the series-coils; and 
finally, when the displacement of cam, drum and slider has 
been carried to such a point, that on an average the arm H 
hits the boom G, as frequently as it misses, there will be no 
further displacement of the cam, drum and slider, which will 
now execute small excursions on either side of a mean 
position as before. Quite similarly, a reduced demand for 
heating current will cause the hits to become more frequent 
than the misses and to remain so until a sufticient displace- 
ment of the cam, drum, and slider in the negative direction 
has been effected, when the hits and misses will once more 
occur with equal frequency on the whole. 
Phil. Mag. 8. 6. Vol. 7. No. 40. April 1904. 2 
