Thermoelectric Properties of Crystalline Bismuth. 327 



force to be at right angles to the current of heat, the former 

 being in the direction of the X axis, the latter in the 

 direction of the Z axis. The following cases may then be 

 investigated : — 



I. For the Longitudinal Effect. 



1. The chief crystallographic axis OA in the direction 



of the current of heat (fig. 1). No new case 

 appears on turning the crystal round OZ through 

 90° a a). 



2. The chief axis OA at right angles to the current of 



heat but perpendicular to the lines of force (fig. 2). 



3. The chief axis OA at right angles to the current of 



heat but parallel to the lines of force. This is 

 obtained by turning OABC (fig. 2) about the Z 

 axis through 90°. 



II. For the Transversal Effect. 



Although 1, la, 2, and 3 are here different cases, only 

 1 and 2 can be investigated since the plate admits of the 

 effect being measured only in the direction of its breadth and 

 not in the direction of its thickness. 



III. For the Thermoelectric Force. 



Along and at right angles to the direction of the chief 

 crystallographic axis. 



§ 4. The Apparatus. — The apparatus was constructed so as 

 to avoid soldering to the crystal. Fig. 3 shows diagram- 

 matically a vertical longitudinal section through the centre at 

 right angles to the plane of the crystalline plate. A is a cylin- 

 drical brass vessel, 10 cm. high and 3' 7 cm. diameter, covered 

 on the outside with felt, and serves to contain the freezing 

 mixture. To its base is soldered a rectangular piece of 

 copper B, 1*5 cm. long, 3'5 cm. wide, and 6 mm. thick. 

 C, I), and E are pieces of copper of the same dimensions as 

 B. F is of copper, and is 3*5 cm. wide, 1*6 cm. square cross 

 section ; through it is bored a hole Gr, 5 mm. radius, which 

 serves to carry a current of water at the ordinary temperature. 

 The piece is soldered to F. Screws pass through the piece 

 of wood H into the coppers B and 0, and maintain them in the 

 position shown in the figure (p. 328) . The pieces D and E are 

 each provided with three screws a, h, c : a', I/, c 1 ; shown in 

 rig. 4, which is a view in the direction of the arrow fig. 3. 

 b, c ; l/ y c' screw into corresponding holes in the coppers B 

 and C. By this means the crystal plate K could be clamped 

 between D, B, and E, C, the screws a and a' adjusting the 



Z2 



