186 
MR. C. V. BOYS ON THE RADIO-MICROMETER. 
rapidly when passing e; hence, the up current at cl and the down current at h will 
each be urged on in the same direction. If, however, the cross started in the other 
direction, the side nearest the south pole would become the hottest, the current would 
pass the other way, and the reversed motion wmuld still be kept up. Thus, which¬ 
ever way it moves, it is kept moving in the same direction. If, however, the heat is 
applied on the opposite side, the first current produced is in a position of stable 
equilibrium ; hence, the circuit shows no disposition to move unless there is want of 
symmetiy, when it moves 45°, and then the equilibrium is absolutely stable. Which¬ 
ever way the circuit is now made to turn, the direction of the currents will be such as 
stop the motion, for the same reason that in the previous case they maintained it. 
In conclusion, the principal advantages in the instrument the development of which 
is described in this paper are :— 
Extreme quickness and sensibility. 
Freedom from extraneous thermal and magnetic influence. 
The sensibility can be varied at wall. 
The instrument may be made dead beat, or its logarithmic decrement may be 
varied at will. 
By the use of the quartz fibre, difficulties caused by the uncertain behaviour of 
silk under varying conditions of temperature and moisture—difficulties that 
would be far greater than in the case of a galvanometer—are completely 
obviated. 
On the other hand, a disadvantage inherent in the instrument is that it must, like 
a galvanometer, be fixed in position ; it is inferior to the thermopile or bolometer in 
the ease with which they can be pointed in any desired direction. 
