522 



§ 3. EAhL-e/ect in graphite. The great change in the properties 

 of graphite through even small admixtures appears -clearly from the 

 fact, that with different kinds of graphite the temperature-coefficient 

 of the resistance may even diffei- in sign. 



The material we started from in our experiments was line graphite- 

 powder, such as is used in electro-plating; the powder was first 

 treated with acids and alkali and carefully dried ; the forming of the 

 plate was again carried out in a steel mould. The electrodes which 

 gave some difficulty at first were finally contrived in the following 

 manner: the powder was provisionally moulded to a plate under 

 comparatively low pressure, the stamp was then lifted off. and six 

 small pellets of solder were laid on the plate, after w^hich a high 

 pressure was applied. By trial we succeeded finally in ol)taining 

 suitable plates of Va ^^^^^^- thickness with six point-electrodes of about 

 7, mm. diameter, penetrating through the whole thickness of the 

 plate. 



The infiuence of insufficient drying of the powder was very 

 marked; such plates, as did not come up to standard as regards 

 drying, did not reach their final resistance until the current had 

 gone through for 6 seconds. 



The following table contains the results. It may be specially noted 

 that the lemperature-coefficient is positive and that the HALL-effect 

 falls strongly from 20° K. to 14° K. 



TABLE III. 



§ 4. The U\hL-e/ect in cadmium. Two circular rolled-out Cadmium- 

 plates of 1 cm. diameter were experimented on. The results are 

 found in the following table. 



