1G2 
MR. C. V. BOYS ON THE RADIO-MICROMETER. 
plan and a vertical section tlirongh the dotted line respectively. The central drum 
of iron, which serves to intensify and make more uniform the magnetic field, is shown 
shaded in these figures. 
The copper hoop works in the annular space between the pole pieces and the 
central drum, where tlie field is most intense, while the active bar of antimony and 
bismuth hangs in the large cavity below the drum, where it is exposed to the radiation 
to be measured and where the weakness of the field prevents the diamagnetism from 
giving trouble. The pole pieces are screwed to massive brass plates, which cover the 
upper, front, and lower sides. The ends are left bare, as is the whole of the other 
side, which rests against the flat, ends of a powerful horseshoe magnet. The 
remaining uncovered part between the poles of the magnet is covered with a piece of 
glass, which enables one, when levelling the instrument, to see if the circuit is free, or, 
when directing a spectrum upon the junction, to see that the desired colour is in its 
right place. It also protects the junction from air currents. Through the brass plate 
forming the front of the instrument, exactly opposite the junction, passes a brass 
tube open at the ends. This may carry a long tube, such as Professor Langley used 
in the bolometer, fitted with gradually diminishing diaphragms, which effectually 
prevent air-currents from penetrating into the chamber. It should also carry, just in 
front of the active plate, a vertical slit, so as to confine the received radiation to the 
line of junction and as small a distance on either side as may be desired. A screw 
passing through the front plate holds the iron drum securely and truly in its place. 
The upper brass plate is pierced by a brass tube, about 31 cm. long, in which there is 
a window at the level of the mirror. There is a simple form of torsion head at the 
upper end of the tube which carries the quartz fibre (see fig. 4). 
I should here point out in what respects my instrument differs from that of 
M. d’Arsonval. He makes his circuit of a pair of wires with the twm junctions in 
the axis of motion, one above and one below. The wires are made of palladium and 
silver. The circuit is hung by a fibre of silk, and is directed by the action of the 
magnet on a small piece of iron wire attached to the circuit. One of the junctions is 
protected hy fixing over it the mirror ivliich reflects the beam ofi light on to the sccde. 
The radiation to be measured is concentrated on to the other junction. M. dArsonval 
does not appear to have used for this purpose carefully formed pole pieces, but has 
simply hung the junction between the legs of a vertical horseshoe magnet, using a 
central hollow drum of iron within the circuit. He sometimes uses no central drum, 
but then he places the two wires very much nearer together, forming a long, narrow 
rectangle, which thus has a very small moment of inertia. He speaks of the great 
sensibility and quickness of his instrument, which also is dead beat. 
In the preliminary note on the radio-micrometer Roy. Soc. Proc.,’ vol. 42, p. 191), 
I pointed out in a provisional manner how the instrument may be made as perfect as 
possible by so choosing the length of the rectangle, the thickness of the copper wire, 
the number of turns or of junctions, the strength of the field, or the torsion of the 
