JOULE 287 



His experiments in this case were of a simple description. 

 He measured the current by means of a very simple galvano- 

 meter which he made himself, and which he graduated by 

 means of a voltameter; the resistances were measured by in- 

 troducing a copper resistance taken as a unit, and measuring 

 the current, and then calculating by Ohm's law. The quan- 

 tity of heat was measured by means of a simple water calori- 

 meter; the most refined element of the whole equipment 

 was a very sensitive mercury thermometer. In the case of 

 metallic conductors he mentions, in connection with the de- 

 pendence of the quantity of heat upon resistance, the dis- 

 covery previously made by himself that discharges from the 

 electrical machine produce an amount of heat roughly pro- 

 portional to the resistance of the circuit; but he also carried 

 out some measurements himself. In the case of liquid con- 

 ductors this was done with greater elaboration, and both the 

 development of heat in the elements, as also in electrolytic 

 cells, was investigated at various measured currents and 

 resistances, and found to be in good agreement with the 

 law. 



His measurements were not carried out with great ac- 

 curacy. However, his experiments were clear in character; 

 all subsidiary matters were recognised and taken into 

 account; thus the development of heat taking place in the 

 cells on open circuit was also deducted, and the polarisation 

 voltage in the electrolytic cells, already discovered by Fara- 

 day, was correctly taken into account in measuring their 

 resistance. It is always the case in the first investigation of 

 natural processes, that great accuracy is not of so much im- 

 portance as clarity in the experiments, and a basis of clear 

 conceptions. Accuracy, and a consequent answer to the 

 question whether further hidden phenomena may still be 

 present, comes later, and often best indirectly. 



Thus our present undoubting trust in Joule's law depends, 

 apart from later and more refined direct measurements, 



