On Contact Electricity of Metals. 



tions are for metals in air. In the experiments described in Chapter 

 X, however, the metallic surfaces were protected by solid non- 

 conducting films, and were not in contact with the atmosphere ; 

 nevertheless, temperature-variations were found, which in the case 

 of silver were actually larger than those which took place in air. 



68. One of the copper plates used was hollow and could be 

 filled with water and a thermometer inserted, but with the other 

 plates other methods of measuring temperature had to be adopted. 

 In some cases the temperature was measured thermo-electrically, 

 while in others a simpler and more rapid mode of measurement was 

 used, which, though not very accurate, is quite reliable within 

 certain limits. 



69. By touching the back of the plate I found that its tempera- 

 ture could be judged as "tepid," "slightly warm," "warm," "very 

 warm," and so on. It was found experimentally that these terms 

 correspond to constant temperatures ; or rather that each term 

 denotes a small range of temperature, the middle point of which 

 may be taken as corresponding to the term. In determining the 

 values of these terms a plate was used in which a thermometer was 

 inserted. One observer touched the plate with the tips of the 

 first and second fingers and judged its state, naming it by one 

 of the terms, "warm," "tepid," &c. ; the other observed the ther- 

 mometer, and the temperatures found to correspond to each term are 

 as follows : 



Cold 16 C. 



Quite cool 24 



Cool 28 



Rather cool 30 



Tepid 35 



Slightly warm 40 



Warm 47 



Very warm 50 



Hot 53 



Very hot 57 



Too hot to touch continuously 63 



Too hot to touch for more than one second . . 73 



This method of measuring temperature is rough and ready, but 

 since the possible errors are within limits of a very few degrees, one 

 only requires to take the average of a considerable number of results 

 in order to arrive at a very fair approximation to the true values. 

 In experiments on contact electricity in air there are so many possible 

 causes of disturbance that extremely accurate measurement of the 

 temperature is of little use, especially if it require that much time be 

 spent over each reading. 



