Electro-thermal Effect in Tourmaline. 221 



thermopile was flattened by being placed between two plane 

 hardened steel plates which were subjected to hydraulic 

 pressure ; it was coated with a thin layer o£ shellac to obtain 

 insulation between neighbouring wires. The plates and 

 thermopile were fitted between the two horizontal plates of 

 a condenser in a vulcanite box 1 cm. high and 12 cm. in 

 diameter, the end-wires of the thermopile being brought 

 out through the curved surface of the box. 



With the d' Arson val galvanometer employed, one scale- 

 division corresponded to 1*83 x 10~ 8 ampere, or, since the 

 resistance of the galvanometer and thermopile amounted to 

 8*59 ohms, to 1*57 x 10 -7 volt. If we take the thermoelectric 

 power of the iron-constantan combination to be 53 x 10~ & 

 volt, then one scale-division represents a temperature-dif- 

 ference between the two sets of junctions of 0*3 X 10" s 

 degree C. 



The small single-plate influence-machine used in the ex- 

 periments had its circuit either open or closed by a cylinder 

 of wood. Between the influence-machine and the plates of 

 the condenser was inserted a paraffin commutator, which also 

 allowed of the short-circuiting of the condenser. The poten- 

 tial-difference between the plates of the condenser — which 

 were placed 1*35 cm. apart — was maintained as high as was 

 consistent with steadiness ; in the majority of the experiments 

 it was estimated at some 30,000 volts ; it might, however, 

 have departed appreciably from this value, as the estimate 

 was based on the spark-length between spheres of 2 cm. 

 diameter, and determinations of this could only be made 

 before and after the actual observations. 



The observations were made as follows : — As soon as the 

 electric field was established, the galvanometer gave a de- 

 flexion whose direction depended on that of the field. This 

 deflexion reached a maximum in about half a minute, and 

 then fell to zero in about four minutes. As soon as this hap- 

 pened, i. e. as soon as no appreciable movement of the spot 

 of light could be noticed, the field was reversed, the maximum 

 deflexion to the other side of zero observed, and so on. The 

 following is such a series of observations ; the scale-divisions 

 were 2 mm. ones (p. 222). 



The extreme two columns on the right are obtained by 

 subtracting the immediately preceding zero reading from the 

 maximum elongation. 



Such a set of observations having been obtained, one or 

 other of the two pairs of plates was reversed, so that now all 

 the plates had their analogous poles pointing in the same 

 direction. Under these conditions no deflexion was observable 



