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XXXIX. The Zeleny Electroscope. By Fkank Horton, 

 Sc.D., Professor of Physics in the University of London *. 



THE Physical Review for 1911 (vol. xxxii. p. 581), 

 contains a description by Prof. John Zeleny of a 

 new type of gold-leaf electroscope, especially suitable for 

 lecture experiments with ionization currents. The novel 

 feature of the instrument is that the ionization current is 

 measured by the period of vibration of a gold leaf instead of 

 by its rate of motion. The gold leaf is suspended from an 

 insulated support, with its plane at right angles to the plane 

 of a metal plate which is maintained at a constant high 

 potential by means of a battery of small cells. The leaf is 

 thus attracted to the plate, charged, and repelled. If the 

 charge is then caused to leak away from the gold-leaf system 

 [e.g. by an ionization current), the leaf falls towards the 

 plate and is attracted up to it and again repelled. This 

 process is continually repeated, and the period of vibration 

 of the leaf gives a measure of the rate of leak. The leaf is 

 thus automatically recharged, and the observer has merely to 

 time the vibrations. This can be done with considerable 

 accuracy, for, as is pointed out in the paper, the leaf being 

 " edge on " to the plate, there is very little resistance to its 

 motion through the air, and it is attracted up to, and repelled 

 from, the plate with a sudden jerk which gives a sharp 

 beginning and ending to each period. 



The author has recently been using a Zeleny electroscope 

 for lecture and research purposes, and has made a slight 

 modification in the original design. The instrument is dia- 

 grammatically represented in the figure. The dimensions 

 and chief particulars of the design are exactly as described 

 by Prof. Zeleny, but there is a difference in the method of 

 suspension of the gold leaf. In the original instrument, near 

 the upper end of the leaf, two cuts are made at the same level 

 on opposite sides of the leaf and nearly meeting in the middle. 

 and the lower, and main, portion of the leaf is turned at right 

 angles to the small upper portion, which is attached to the 

 supporting rod. I found that this method of suspension was 

 rather difficult to perform, and the method indicated in the 

 diagram was tried and was found to work successfully. The 

 leaf is attached to a light aluminium wheel which was made 

 in the following manner : — a circular piece of thin sheet- 

 aluminium 1 cm. diameter was cut, and three holes (3 mm. 

 diameter) were drilled in it, in symmetrical positions, to 



* Communicated by the Author. 



