436 On Radiation from Dull and Bright Surfaces. [June 16, 



vacuum has been made. Flexible copper electrodes, ee, are silver- 

 soldered to the loops at the ends of the copper springs, and to 

 multiple platinum wires which are sealed into the glass tube at A 

 and B. Fine platinum wires, pp, are attached to the main wire, and 

 are brought through the sides of the glass tube ; and these serve as 

 potential testing electrodes. The two tubes and their fittings are, as 

 has been said, perfectly similar in every respect, except that one 

 platinum wire is covered with an extremely thin coating of lamp- 

 black. 



The two tubes were attached to a glass fork, and were simul- 

 taneously exhausted with the Spree gel pump down to about two 

 millionths of an atmosphere, all the well-understood precautions as 

 to drying, &c, being carefully attended to ; and they were then at 

 the same moment sealed off from the pump. The length of the tubes 

 AB is 22 inches over all; and the internal diameter of the tubes 

 J inch. The distance between the potential electrodes pp is 15 inches 

 (38*1 centimetres). The diameter of the platinum wire ah is 0*022 

 inch (0*0599 centimetre). On testing the resistance of the two 

 platinums between the potential electrodes, cold and at the same 

 temperature, it was found to be the same for both to less than one 

 one-thousandth part of the resistance of either of them. 



The tubes having been prepared as described above, they were 

 connected in parallel arc to a battery of six secondary cells in series, 

 a variable platinoid wire being added in series with the tube con- 

 taining the bright platinum, in order to regulate its current ; and a 

 rheostat designed for carrying strong currents was used to control 

 the whole. The connexions will be readily understood from a glance 

 at fig. 2. 



Fig. 2. 



With the rheostat and the variable platinoid wire the two plati- 

 nums were then brought to the same incandescence (as judged by 

 the eye) at various brightnesses from just visible redness up to nearly 

 white heat; and the resistances of the platinums between the potential 



