182 Mr Laws, Experiments on the Thomson Effect 



The resistance of this circuit was about 1*5 ohms and the 

 galvanometer could easily be arranged to give a deflection of 

 1 division of a scale reflected in a telescope for a current of 

 10 -7 amperes. 



The thermo-electric power of the couple was found to be 

 about 33 x 10~ 6 volts per 1° C, so that a deflection of one division 

 corresponds to a change of temperature of ^lo^h degree centigrade. 



The current thus sent through the galvanometer by the e.m.f. 

 of the couple was compensated by adj usting a current in the other 

 set of coils of the galvanometer. 



This compensating current was obtained from a single secondary 

 cell joined up with a set of known resistances. From a part R 2 

 (fig. 1) of the main circuit a current was taken off to pass through 

 the galvanometer and high resistance _R 3 . 



The required magnitude of this compensating current was 

 obtained by first choosing a suitable value for R 2 , then adjusting 

 first R t and finally R 3 ; the current is then calculated from the 

 E.M.F. of the cell and known resistances. 



Each galvanometer circuit included a key K 2 consisting of a 

 vertical copper rod dropping into a mercury cup; the two rods 

 were insulated from each other and connected to the same movable 

 upright worked by a string over a pulley, their lengths being 

 adjusted so that they touched the surfaces of the mercury in the 

 cups at the same instant. 



By this means the compensating current could be adjusted so 

 that the galvanometer gave no deflection when the upright was 

 released and the circuits completed. 



This value of the compensating current is then proportional to. 

 the difference of temperature between the points of the rods to 

 which the junctions are attached. 



The method of carrying out an experiment is then as follows : 



The hot bath is filled with boiling water and this is kept boil- 

 ing either by means of a current in a coil of wire placed inside the 

 bath or by means of a small gas flame underneath ; the cold 

 bath is filled with melting ice, the ice being prevented from 

 coming into contact with the rods by means of wire gauze cages 

 surrounding their free ends. 



When the temperature of the rods has become steady through- 

 out a constant current — 3 or 4 amperes — is passed through them. 



The temperature again becoming constant, as indicated by the 

 constant value of the current in the junction circuit, the difference 

 of temperature of the corresponding points of the two rods — or, 

 rather, the current in the compensating circuit necessary to 

 counterbalance the current in the galvanometer due to this 

 difference of temperature of the junctions — is observed. 



The current through the rods is then reversed, when, by virtue 



