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XIX. A Short-Period Electrometer, and its use in Determining 

 the Frequencies of Slow Electrical Oscillations. By E. 

 Taylor Jones, D.Sc, Professor of Physics in the 

 University College of North Wales, Bangor *. 



[Plate IX.] 



IN the " Rapports presentes au Congres International de 

 Physique/' vol. iii. (1900), a description is given by 

 Blondel of various methods which have been employed for 

 the direct demonstration of varying currents and potential 

 differences. Most of the instruments designed for this purpose 

 (oscillographs, rheographs, &c.) depend upon electromagnetic 

 action, a fine wire or strip carrying the current, a small bar 

 or strip of soft iron, or a beam of cathode rays, being deflected 

 by an electromagnetic force proportional to the current. 

 The Braun tube may also be used as an electrostatic instru- 

 ment, the beam of cathode rays passing between two plates 

 connected to the source of varying E.M.F. and being deflected 

 by the electrostatic force between them, So far as I am 

 aware, this is the only electrostatic method hitherto employed 

 for rapidly varying electromotive forces, and it appears to 

 be difficult by this method to obtain a very clearly defined 

 wave-curve, while according to Zenneckf the Brann tube 

 method has not hitherto been found to give very accurate 

 quantitative results. 



For the study of slow high-potential electrical oscillations 

 an electrostatic method would seem to require simpler 

 apparatus than most of the electromagnetic methods, and to 

 have the advantage that no additional self-inductance is 

 introduced into the circuit. The experiments described in 

 this communication constitute an attempt in this direction. 



(1) Apparatus. 



A piece of phosphor-bronze strip, S (fig. 1), is soldered at 

 one end to a terminal on an ebonite pillar, P, and at the 

 other to a small spiral spring, L, attached to a screw. A 

 second ebonite pillar supports the screw which can be drawn 

 through by a nut, thus allowing the tension of the strip to be 

 varied. The strip rests horizontally against two vertical 

 glass rods, P, about 2 cm. apart. To the middle of the 

 strip is attached a small mirror, M,of very thin silvered glass, 

 rectangular or triangular in form, and one or two square 



* Communicated by the Author. 



t Elehtromaqnetische Schwingungen u, clrahtlose Telegraphie, p. 360 

 (1905). 



