Magnetic Fields on Metallic Resistance. 901 



further study of this side of the problem seem well worth 

 while. Then, too, only the strongly magnetic metals have 

 been carefully examined in longitudinal fields; little is known 

 of the behaviour of the para- and diamagnetic metals. The 

 experiments to be described were undertaken with the idea 

 of making a comparative study of the longitudinal and 

 transverse effects in strong and weak fields, and for some 

 other metals besides the ferromagnetic ones. 



The apparatus used for measuring the resistances and 

 changes of resistance was essentially a Wheatstone's bridge. 

 Coils of constantan wire, each of 3 ohms resistance, made up 

 three arms of the bridge; in the fourth was inserted the 

 metallic conductor to be examined. The coils w r ere fixed to 

 a hard rubber base and immersed in oil to maintain constant 

 temperature. Variable resistances were arranged as shunts 

 around one arm, and by changing one of these resistances by 

 a fairly large amount bahince could be secured when the 

 conductivity of the metallic specimen changed only very 

 slightly. When the effect was too small for this method, 

 deflexions of the galvanometer were observed, and change of 

 resistance computed by calibrating the galvanometer. 



The galvanometer was of the four-coil astatic needle type, 

 provided with a triple shield of soft iron, so that the strongest 

 outside field produced no appreciable effect. It was connected 

 up to have a resistance of 3 ohms, and could be made sensitive 

 to 5 X 10 ~ 10 ampere. 



For producing the magnetic field either an electromagnet 

 or a solenoid could be used. The solenoid was 1 metre in 

 length, had 33 turns per centimetre, and could produce fields 

 as large as 2200 c.G.s. units. It was constructed around a 

 brass tube, inside of which a smaller tube was fixed. The 

 space between the two tubes was kept full of cold running- 

 water, and the metal under examination was contained inside 

 the inner brass tube. This inner tube could be filled with 

 oil, so that the heating effects of the solenoid current were 

 reduced to a minimum. 



The electromagnet produced fields as large as 10,000 when 

 the pole-pieces were about 4 centimetres in diameter and 

 2 centimetres apart. This was the distance used in most 

 cases. The pole-pieces were fitted through apertures in oppo- 

 site sides of a brass box, and this box was filled with water 

 or oil, so that when the metal to be examined was placed 

 between the poles it was immersed in the liquid. Stirrers 

 driven by a fan-motor could be used to keep the liquid in 

 motion, so that little trouble arose in this case from tempe- 

 rature variation of the metal specimen. 



Phil Mag. S. 6. Vol. 22. No. 132. Dec. 1911. 3 



