490 M. H. Weber on the Heat-conducting Power 



for shortness we put 



we obtain 





v„-v=R ,rz^ . . . . (11) 



tan (^ 

 1 4- fl sin cj)'" 



Accordingly, when the quantities R and a are known, and <f) and 

 U observed, the temperature of the middle is thereby given. 

 The quantity a had been determined after the method given by 

 Poggendorff*. As the above galvanometer (constructed by 

 myself) had perfectly regular coils, and care had been taken to 

 secure a good centring of the multiplier, the values of a, calcu- 

 lated from various angles of torsion of the multiplier, exhibited 

 a satisfactory accordance. Yor example, a set of such observa- 

 tions gave for a : — 



-0-6328 



-0-6259 



-0-6265 



-0-6316 



— 0-6274 



Mean . . -0-6288 



A second set gave similar results. For the following obser- 

 vations the value 



fl=-0-63 



is perfectly sufficient. The quantity R depends on the resist- 

 ance W of the circuit, and therewith upon the temperature of 

 the galvanometer-wire. An alteiatiou of this temperature could 

 have been easily reckoned according to the known law of the va- 

 riation of the resistance with the temperature ; but as the tem- 

 perature of the wire cannot be accurately determined, it was 

 preferred to determine R specially before each series of obser- 

 vations. 



At the beginning of each series of experiments, before the ends 

 of the bar were exposed to a heating or cooling, the temperature 

 of the bar was that of the surrounding air. To control this, the 

 conduction to the middle wasmserted, so that the needle showed 

 no deflection. A vessel with heated oil was now approached 

 from beneath to the points and the thermometer between them 

 (fig. 5} until they were in the centre of the oil. The vessel 

 containing the oil was suspended free in a cylindrical vessel of 

 wood of 1 inch thickness (fig. 6), upon which \\as a wooden lid 

 with a circular aperture. Through this the points and thermo- 



* Pogg. Ann, vol. Ivii. p. 324. 



