( 808 ) 
say that we had previously verified the absence of magnetic pro¬ 
perties from the ebonite by means of an apparatus after Curie in 
which we utilised the attraction in a non-uniform field. 
For the measurement of the field there were placed in circuit 
with the galvanometer the coil on the ebonite cylinder, a manganin 
resistance to regulate the sensitivity, a secondary coil of 500 turns 
fitting round the standard solenoid, and finally, an electromagnetic 
arrangement which could be used as a damper if desired. We 
also allowed for the very small deviations from the law of pro¬ 
portionality between the deflections of the galvanometer and the 
quantities of electricity, which had been determined for the galvano¬ 
meter (one of the Deprez-d’Arsonval type) by a previous investigation. 
The solenoid was constructed with the greatest accuracy by winding 
bare copper wire on a core of white marble 1 ). 
The standardisation of the galvanometer was made by reversing 
the current in the solenoid; the observations made by withdrawing 
the coil from the field always took place between two standardisations 
of the galvanometer; there was, however, no change in the galvano¬ 
meter constant to be observed. The corrections and precautions 
necessary in obtaining the strengths of the current are the same as 
in the case of the balance, and have already been described. The 
final result of this ballistic method is 
The relative difference between this and the value given by Cotton’s 
balance is therefore 0.0012; and this can be neglected especially 
when one remembers that almost every one of the numerous meas¬ 
urements necessitated by the one method as much as by the 
other, beginning with the adjustment of the field by means of the 
ammeter, is accurate only to 0.0005. It may be useful to comment 
here upon a particular point that increases the difficulty of obtaining 
this agreement and therefore enables us to rely more upon the 
correctness of the numbers which we have obtained. The equation 
for Cotton’s balance involves the strength of the current in the 
denominator , while this magnitude in calculating according to the 
ballistic method occurs in the numerator ; a systematic error there- 
!) For the dimensions and the description of the solenoid and galvanometer 
see: P. Weiss, Mesure de 1’intensite d’aimantation a saturation en valeur absolue ? 
Arch. Sc. phys. et nat. February 1910, J. de phys. May 1910, 
