674 



PEOCEEDINGS OF THE AMERICAN ACADEMY. 



avoided in setting up and using this instrument. It was placed on a 

 heavy iron pillar rising from the cement floor of a basement room. 

 The outside of the wooden case was covered with tinfoil, and a par- 

 tition of roofing tin, 2 m. high by 1.3 m. wide, was placed between the 

 instrument and the tube and was put to earth. A hole 1 cm. in 

 diameter was bored in the partition to give passage to the rays. As 



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ElGUKE 1. 



Figure 2. 



an indication of the efficacy of these precautions against electrostatic 

 disturbance, it may be stated that when powerful sparks were allowed 

 to pass to the partition from the coil used to drive the tube, the zero 

 reading of the instrument was not in the least affected. It seemed 

 that the magnetic field about the suspended system exerted a con- 

 siderable directive force upon the latter, notwithstanding the soft-iron 

 block, and direct experiments confirmed this belief In order, therefore, 

 that not only the sensitiveness of the instrument but its zero reading 



