694 PROFESSOR THOMSON ON THE ELECTRO-DYNAMIC QUALITIES OF METALS. 
cotton-thread, was pushed into the central part of the tube, in which it fitted closely, 
and was carefully luted with red lead. After keeping it for several days heated by a 
stove, gutta-percha coolers. A, A, were fitted on it, leaving a length of 6 inches of 
tube between them. Wooden troughs, B, B, were then fitted on outside the coolers, 
and fastened to the ends of a piece of wood, C, C ; straps of thick copper, about an 
inch broad, were bent to form conducting linings for the troughs, their ends turned 
round, firmly fastened to C, C, and brought together at D, D, thus forming con- 
Fig. 8. 
nexions with the electrodes of the commutator (for this part of the arrangement 
see also fig. 9). Two pieces of thermometer-tube, bent to right angles, had their 
short arms rolled with thread, and were pushed into the tube from its ends, as far as 
b, h, leaving spaces ah, ah, each two-thirds of an inch, between them and the stopper 
aa in the middle of the tube, and made air-tight by cement applied at E, E. The 
dotted line represents the space round the tube and its wooden stand C, C, filled 
with cotton-wool. A conducting communication was established between the pla- 
tinum tube and D, D, by pouring mercury into the troughs (see also fig. 9). 
Fig. 9. 
70 . The system of regulating the temperature in one part of the conductor by 
breaking and making the circuit, had been adopted only as a temporary expedient in 
the experiment on the iron conductor (§ 65), in consequence of the failure of a conti- 
nuous regulator which had been fitted up for that experiment. It had the advantage 
