538 
Proceedings of the Royal Society 
4. On a Thermomagnetic Thermoscope. 
By Sir W. Thomson* 
This thermoscope is founded on the change produced in the 
magnetic moment of a steel magnet hy change of temperature. 
Several different forms suggest themselves : the one which seems 
best adapted to give good results is to be made as follows ; — 
(1.) Prepare an approximately astatic system of two thin, har- 
dened steel wires, r b, r b', each 1 cm. long, one of them, t b, hung 
by a single silk fibre, and the other hung bifilarly from it, by 
fibres about 3 cms. long, so attached that the projections of the 
two, on a horizontal plane, shall be inclined at an angle of about 
*01 of a radian (or *57°) to one another. 
(2.) Hang a very small light mirror bifilarly from the lower 
of the two wires. 
(3.) Magnetise the two wires to very exactly equal magnetic 
moments in the dissimilar directions. This is easily done by a 
few successive trials, to make them rest as nearly as possible per- 
pendicular to the magnetic meridian. 
(4.) Take two pieces of equal and similar straight steel wire, 
well hardened, each 2 cms. long, and about *04 cm. diameter ; 
magnetise them equally and similarly; and mount them on a 
suitable frame to fulfil conditions (5) and (6). Call them B B 
and B' B', B and B' denoting the ends containing true north 
polarity (ordinarily marked B), and E B' true south (ordinarily 
marked red). The small letters r, b , r', b' mark, on the same plan, 
the polarities of r b and r' b' . 
(5.) The magnets E B, B ; B', are to be relatively fixed in line 
on their frame, with similar poles next one another, at a distance 
of about 2 cms. asunder; as thus B B . . . B' E', with B B' = 
2 cms. 
(6.) This frame is to be mounted on a geometrical slide upon 
the case within which the astatic pair r b, r' b' is hung, in such 
a manner that the line of E B, B' B' bisects r b, approximately at 
right angles, and that E B B' E' may be moved by a micrometer 
screw through about a millimetre on each side of its central 
position, the line of motion being the line of E B, B' B', and the 
