132 Mr. H. F. Biggs on Decrease in the Paramagnetism 



absorbed hydrogen on the susceptibility of palladium, an 

 effect believed on the authority of Thomas Graham (1) to be 

 a strong increase to the paramagnetism of palladium. A 

 preliminary experiment, however, contradicted Graham's- 

 result, showing that palladium when saturated with hydrogen 

 becomes almost neutral. Since the method adopted is con- 

 siderably modified from that used in previous absolute 

 determinations of susceptibility, the experimental arrange- 

 ments are described in detail. 



II. Method. 

 I. Principle of the Method. 



A disk of palladium foil is placed so that it fits within a 

 small coil (coil A) carried on the arm of a torsion-balance,, 

 which swings in a non-uniform magnetic field. A current 

 passed through the coil is then adjusted so that the force 

 exerted by the field on the current balances the force on the 

 palladium and on the matter of the arm itself. The process 

 is repeated with the palladium removed. The difference of 

 the balancing currents, multiplied by the total area of the 

 coil, gives directly the magnetic moment, M, of the palladium. 

 The intensity, H, of the field at the same spot is deduced 

 from the throw of a galvanometer connected to the same 

 coil when the circuit of the electromagnet is broken, this 

 deduction being based on the comparison between the throws 

 given by another coil (coil B), 



(a) when the circuit of the magnet is broken, 



(b) when the coil is snatched out of the field. 



The mass-susceptibility (that is, the magnetic moment per 

 gram per gauss) is then K TO = M/Hm, where m is the mass of 

 the palladium. 



2. The Electromagnet and its Circuit, 



The magnet used was an " optical magnet " with cylindrical 

 coaxial cores, 7 cm. in diameter, whose distance apart could 

 be adjusted by screw collars. To the ends of these cores 

 were screwed pole-pieces designed to give a field in which 

 the ponderomotive force would be parallel to the axis of 

 symmetry of the field, and roughly constant for all points on 

 or near this axis. This form of field, essential in the 

 preliminary experiment, is also useful in this method, as it 

 renders unnecessary the exact replacement of the disk of 

 foil in the same position in the coil — a difficult matter in the 

 case of palladium-foil, which expands when charged with. 



