AND STRAIN ON THE ACTION OP PHYSICAL FORCES. 
113 
wire, Y, are secured to the clamps at A A and B, the two wires having previously to 
clamping been enveloped in cotton tubes of the same lengths as the wires. As a 
further precaution, the clamps A were well wrapped up in silk, and with the wires 
were enclosed in a cardboard cylinder which fitted neatly into the inner brass cylinder. 
When it was necessary to heat the wires, jackets of several folds of baize were placed 
round the outside of the air chamber and also completely covered the top. 
Fig. 17. 
The connexions with the resistance coils of 100 ohms and the galvanometer were 
exactly as in previous experiments, so that from the disposition of the wires as showm 
in the figure, it will be seen that the influence of change of temperature on the 
connexions would be entirely neglectable. Moreover, the boiler and air chamber, 
which were some distance from the box of resistance coils, were screened from the 
latter, which was covered on the outside with tin foil, and only opened at the moments 
of actual testing for the balance between X and Y. The wires X and Y were in all 
cases 50 centims. long, and so disposed that they occupied the central portion of the 
chamber ; whilst a thermometer, T, served to indicate the temperature. The air 
chamber stood upright on a table, and the lower portion of it, It, was packed with 
sawdust on which the base of the cardboard cylinder rested."' 
* It would have been better to use a cylinder of copper foil instead of the cardboard, as thereby the 
temperature would have been rendered more uniform throughout the length of the wire. 
MDCCCLXXXTII. Q 
