88 



ELEMENTS OF LABORATORY WORK 



when placed in the liquid of a cell, will indicate a rise of tem- 

 perature when the circuit is closed. At the same time, another 

 thermometer, in contact with another portion of the circuit, 

 may indicate a rise in temperature, which will not necessarily 

 be the same as that taking place in the cell. 



The extent of the change of temperature may vary very 

 much in different portions of the circuit ; but the changes of 

 temperature are alike in those portions where the dimensions 

 and quality of the matter investigated are precisely alike. 



At the same time, the change of temperature varies with 

 the stress in the circuit, or, as it is called, the strength of cur- 

 rent ; and this change takes place proportionally throughout 

 the circuit. The electric stress may readily be caused to 

 diminish by adding to the circuit a body of low conductivity, 

 such as a long, thin wire. A comparison may then be made of 

 the relative changes of temperature in the same substance. 

 The variation of the stress may be observed by the deflection 

 of the magnet of a galvanometer, which is made to form part 

 of the circuit. It is advisable to wrap a certain length of 

 wire round the bulb of a thermometer and place them both 

 in a vessel of water. The wire is then connected with the 

 rest of the circuit. In this way a change of temperature is 

 readily observed. The rise of temperature will be found to 

 depend upon the time during which the circuit is closed. 



The conductivity of metals diminishes with a rise in tem- 

 perature. The conductivity of saline solutions, on the other 

 hand, increases rapidly with a rise in temperature. 



Approximate Relative Conductivity at C. 



