180 Mr Laws, Experiments on the Thomson Effect 



In this way it was possible to obtain uniform solid rods with a 

 perfectly smooth surface. 



The glass was then removed from the rods and these annealed 

 by heating in an oil bath to a temperature as near the melting 

 point as possible and very slowly cooling. 



Owing to the greater expansion of bismuth on solidifying, 

 glass tubes could not here be used as moulds ; in this case a mould 

 of slate was employed. 



This was obtained in two pieces by clamping together two long 

 slabs of slate rectangular in cross section and drilling a hole of the 

 required diameter along the length of the block with its axis 

 coinciding with the line down the centre of the plane of junction. 



This mould was placed inside a cylinder, around which a coil 

 of German-silver was wound, and heated to about 250° C. by means 

 of a current in the coil, when the liquid metal was poured in. 



By means of an iron clamping arrangement the two slate 

 blocks could be kept in close contact until the molten metal had 

 solidified, and then by separating the two halves of the mould the 

 rod could be withdrawn and annealed. 



The method adopted for measuring the " specific heat " is that 

 employed by Haga in his experiments with mercury and platinum* 

 and similar to that originally used by Le Rouxf. 



It consists essentially in comparing the change of temperature 

 produced when the direction of a current, passing along a tempera- 

 ture gradient, is reversed with the rise in temperature produced at 

 the same point by the passage of a current in the rod when this is 

 at an uniform temperature throughout. 



In this latter case the amount of heat evolved per unit length 

 per unit time is known from Joule's law ; so that, assuming that 

 the amount of heat produced is proportional to the change of 

 temperature, the amount of heat evolved or absorbed in the former 

 case may be calculated. 



The "specific heat," a, is then obtained from the relation 

 H=Cadd, where H is the quantity of heat evolved or absorbed 

 per unit time by a current G in passing between two sections 

 whose difference of temperature is dd. 



To increase the magnitude of the effect to be measured and 

 also to eliminate as far as possible any errors due to the want of 

 homogeneousness of the material two rods were used. 



These were placed parallel at a distance of 12 cms. apart with 

 one end of each passing through an indiarubber stopper into a 

 bath in which water could be kept boiling and the other end in a 

 bath which might contain melting ice or through which a current 

 of cold water could be circulated. 



* Ann. de I'ecole polytechnique de Delft, i. p. 145, 1885, in. p. 43, 1887 

 t Ann. de Chim. et de Phys. x. p. 258, 1867. 



