62 Messrs. Boys, Briscoe, and Watson on the 



duced by one Daniell cell when the external resistance is 

 115 ohms, and the working resistance 38 ohms. 

 Therefore the heat produced per sec. 



38 1" ■ . 



X -T7* calories, 



~ 115f 2 " 4-2 



= •000685 calorie; 



or it would take 21 minutes 20 sec. to produce one calorie. 



The steady E.M.F. between the terminals of the wire 

 which produces the same effect as the oscillations is *33 volt. 

 Mr. Gregory gives *5 volt as the equivalent E.M.F. in his 

 experiments. Though the conditions were not sufficiently 

 similar to make the numerical results strictly comparable, 

 nevertheless the fact that the figures obtained are of the same 

 order goes to show that the expansion observed by Mr. 

 Gregory was a true expansion due to heat, and that his results 

 were real. 



In calculating from the heat developed the mean square 

 of the current produced in the wire by the electrical 

 oscillations, it is necessary to take into account the fact that 

 these rapidly alternating currents are not uniformly diffused 

 throughout the cross section of the wire, but only pass 

 through a thin stratum at the surface. On this account the 

 resistance which the wire offers to these rapidly alternating 

 currents is greater than that which it offers to steady currents. 



Lord Rayleigh * gives a formula for calculating the effec- 

 tive resistance of a wire of round section to an alternating 

 current of given frequency. It is 



where IT is the effective resistance ; R the resistance to 

 steady currents in C.G.S. units ; p2ir times the number of 

 oscillations in one second ; I the length of the wire ; and fj, 

 the magnetic permeability of the substance of which the wire 

 is composed. 



In the following calculation one of the wires only is con- 

 sidered. 



The length of the primary oscillator is 109 centimetres, 

 and therefore the wave-length of the radiation given out is 

 218 centimetres. If n = number of oscillations per sec, then 



n= ~ = 1-376 xlO 8 . 



A, 



* Phil. Mag., May 1886, p. 388. 



