404 DR. C. H. LEES ON THE THERMAL CONDUCTIVITIES OF SOLIDS 



Hence, 



and the total heat imparted to the discs 



= H = irrVi jv M + *>c + 7- (MM + *s * M + % + * uVu + ^U + 

 Eliminating & between these equations, we have 



r -4 



For the discs used wi M c M = 3 '2, m s c s = 1'8, Imc 11 '5. and during the experi- 

 ments II was about '22, and the value of the fraction on the right-hand side of the 

 equation, '4. Hence, for the error introduced by neglecting dv/dt to be less than 

 5 per cent., we must have 



or 



dv/dt < '00045, 



so the change of temperature in 1 minute ought not to exceed '03 0. 



No observations in which this rate was exceeded have been used in what follows. 



Loss of Heat along Thermo-Wires. 



In the above theory, the heat conducted away from the discs by the wires of the 

 thermu-j unctions, has been taken into account by adding a small correction to the 

 thickness of the discs. To determine the amount, of this correction for any wire, 

 we write 



-v x 



v = v e v * 



where v is the temperature excess at a point of the wire distant x from the disc, 



p = perimeter of section of the wire. 



q = area 



k = conductivity of the wire. 



li = emissivity 



