108 
MR. W. CROOKES ON REPULSION RESULTING FROM RADIATION. 
Received October 16, 1878. 
ACTION OF HEAT APPLIED INSIDE THE RADIOMETER 
425. In a previous paper it has been shown that phenomena, feeble and contradictory 
when caused by radiation external to the bulb, became vigorous and uniform when the 
radiation was applied internally by the agency of an electrically heated wire (336 to 
384). It was hoped that some of the more obscure phenomena shown by the deep 
cups with movable screens in front (400, et seq.) might be intensified if set in action 
by a hot wire. 
Fig. 17. 
An apparatus was made similar to the one described and figured in a former paper 
(360, fig. 32, C D), but instead of the movable fly being of sloping mica vanes, it was 
composed of deep metallic cups with mica screens in front, capable of being brought by 
tapping any desired distance from the cups. This fly is similar to the one shown in 
fig. 8, paragraph 400, the complete instrument being shown in the annexed cut, fig. 17. 
It consists of a wide glass tube, a, b b, sealed off and blown round at a, and drawn off 
narrow at the end b b. Inside is a stem c d d, with branches. A disk of silver-flake 
mica, e, lampblacked on the upper side, rests on the platinum wire ring, the ends of 
which are joined to thicker platinum wires passing through the glass at ff The disk e 
is pierced in the centre, through which passes the stem c, carrying a needle point on 
which the compound fly g rotates. Above the fly is a flat disk of clear mica, In, having 
a glass cap in its centre, and rotating on a needle point. The fly and the clear mica 
disk are supported independently of each other on separate needle points held in the 
glass rods c, d d. The tube i connects the apparatus with the mercury pump, and 
when the apparatus is exhausted to the requisite degree it is sealed off at j. The wire 
is heated by two Grove's cells connected with f f the current passing through a 
contact key, resistance coils, and galvanometer, to keep it constant, as explained in a 
former paper (359). 
The plan of the adjustable fly with aluminium cups and mica screens is shown in a 
previous drawing (fig. 8, par. 400). The different positions of the screen are also 
