192 



On the "Radio-micrometer." 



[Mar. 24, 



paint the circuit with a solution of a salt of iron until it is mag- 

 netically neutral. 



Mr. Cunynghame suggested to me, when I explained to him my 

 views, that a rotating instrument might possibly be made like 

 Orookes' radiometer. After some trouble I have devised and made a 

 most simple rotating pile. It consists of a cross with bismuth arms 

 and an antimony centre. To the end of each arm is soldered a piece 

 of copper wire, the four wires being parallel to one another, and at right 

 angles to the plane of the cross. The four free ends of the wires are 

 soldered to a ring of copper wire parallel to the cross. If this is 

 balanced on a point between the poles of a permanent magnet, and if 

 radiant heat is allowed to fall on the right hand side of the cross, 

 looking from the north to the south pole, the cross will at once begin 

 to oscillate, making larger and larger oscillations until it rotates, 

 which it will do indifferently in either direction. If the heat falls 

 upon the left hand side, any motion that it may have is at once 

 arrested. It follows that if the source of heat is removed and the 

 cross is turned mechanically, the right hand side will be cooled and 

 the left warmed. 



If the cross is made with antimony arms and a bismuth centre, 

 then what was true of the right hand side is now true of the left, and 

 ■vice versa. 



Instead of a cross and four wires, 1 think an antimony disk, with 

 many pieces of bismuth and many wires or with a ring of bismuth, 

 and many wires forming a sort of drum armature, would give a 

 better result ; but the one described with four arms rotates rapidly 

 when the spark at the end of a blown- out match is held near it. 



We have in this rotating pile, which might be called an electric 

 radiometer, an electromagnetic motor, which is almost an exception 

 to the axiom that such an engine cannot be made without sliding or 

 liquid contacts. 



I am now working out fully the conditions of maximum sensibility, 

 and hope to communicate shortly a paper to the Royal Society, in 

 which they are discussed, and at the same time to show an instrument 

 of the best construction and some results obtained with it. 



Note added March 23, 1887. 



I have found, both by calculation and experiment, that the move- 

 ment of the circuit becomes perfectly dead beat with a field of a little 

 over 1000 units ; therefore advantage is gained by using more than 

 one junction, as a stronger field can then be used before the motion 

 becomes dead beat. 



I have spoken of a fine glass fibre as a torsion support. Since this 



