MR. C. V. Bors ON THE RADIO-MICROMETER. 
185 
a deviation of 45 mm. every time a small shutter close to the candle was pulled on 
one side with a piece of cotton, nothing else being allowed to move. Making a strict 
comparison between this result and those referred to in the preliminary note, this 
would give 1530 feet as the distance to which a halfpenny might be taken from a 
candle flame before the heat which it would receive would, if concentrated on the 
sensitive surface, be too small to produce a deflection of ^ mm. This flgure is con¬ 
siderably in excess of that obtained before, even though the flbre is 10 instead of 
38 cm. long, the scale is only 30 inches from the screen, and a deflection of ^ 
instead of mm. is here assumed as the least that could be observed with 
certainty. 
With regard to the rotating pile described at the end of the preliminary note, 
I ought to say that something very similar is mentioned in Noad’s ‘Electricity and 
Magnetism,’ but I was not aware of this at the time of publication. There is, how¬ 
ever, a curious difference, which is worth pointing out. So far as I have been able to 
learn, the wire frames described in Noad’s book only rotate one way when on one 
pole, and the other way when on the other pole of a magnet. Now, my arrangement 
will not rotate at all when placed over a pole; it will only rotate when between two 
poles, and then it will go either way when the heat is applied on one side, but will be 
prevented from moving when the heat is applied on the other side. Though the 
matter is of little importance, I may perhaps explain the reason for the peculiar 
behaviour of my arrangement. 
The cross seen in fig. 8 is made with. bismuth arms and an antimony centre. At the 
ends of the arms four copper wires, a, b, c, d, are soldered at right angles to the plane 
of the cross, and lower down to a ring of copper. The whole is balanced on a point 
between the poles of a magnet, and is free to turn. 
If heat is applied to the point e, an up current in the wire is produced at c, and a 
down current at a. Hence, the position of the cross is one of unstable equilibrium. 
Whichever way the cross begins to move, it will be kept moving in this direction. 
Suppose it to start in the direction of the arrow, that side of the antimony centre 
which faces the north pole will become the hottest, though it is gaining heat most 
MDCCCLXXXIX.—A. 2 B 
