58 Messrs. Boys, Briscoe, and Watson on the 



to the bottom. They were connected together at the top, and 

 at the bottom were brought out and connected to two binding- 

 screws fixed on the ring D (fig. 5). By means of this 

 wire, whose resistance was measured, a known amount of 

 heat could be developed in the tube by passing a current of 

 known intensity. The current was, in all the experiments, 

 supplied by a single Daniell cell, which contained in its 

 circuit, beside the wire of the instrument, a resistance-box 

 and a key. 



The clock was kept working for about half an hour before 

 the commencement of an experiment, in order to equalize the 

 temperature within the tube ; and then a fresh piece of phos- 

 phorus was introduced. When the phosphorus index was 

 quite steady, and approximately at the centre of the scale, the 

 reading was taken (zero-reading), then resistance was un- 

 plugged in the box, and the current passed till the index had 

 again come to rest. The current was then stopped, and 

 another zero-reading taken. 



In testing the different tubes the largest resistance which 

 would give a readable deflexion was in each case ascertained. 

 The resistance of the cell and the connecting-wires could be 

 neglected in comparison with the external resistance. 



In the different tubes the total heat developed was not the 

 same when the same current was passed, as the wires were of 

 different lengths. This, however, was not taken into account 

 in comparing the tubes together, as for the measurement of 

 the electromagnetic radiation it was necessary to get the 

 greatest possible sensitiveness per centim. of the wire. 



It is hardly necessary to give particulars of the different 

 tubes experimented upon. It is sufficient to say that, while 

 the length of the tube was of little consequence, it was found 

 that a wide tube was much more sensitive than a narrow one. 

 The results obtained in a particular case are given. 



Tube No. 3 :• — Length 82, diameter 5 centimetres. Re- 

 sistance of wire 29 ohms ; outside resistance 500 ohms. 

 Deflexion 1 millimetre. 



To calculate the amount of heat corresponding to this 

 deflexion, we have, taking the E.M.F. of a Daniell cell as 

 1 volt, 



Heat developed per sec. = -^r- 



29 i 



- -^ x j^ = -000,024 calorie 



529f 4-2 



over the whole length of the tube, or '000,000,3 calorie per 

 centimetre of the tube. 



