106 Mr Laby, A string electrometer. 



A string electrometer. By T. H. Laby, B.A., Research Student, 

 Emmanuel College, Cambridge, Joule Student of the Royal 

 Society. 



[Read 25 January 1909.] 



The minute force, 10~^ dynes, that is sufficient to perceptibly 

 deflect a stretched silvered quartz fibre, is of the same order of 

 smallness as that acting on the gold leaf of the most sensitive of 

 the usual electroscopes. This suggested that the fragile and irregu- 

 larly shaped gold leaf might be replaced with advantage by a 

 silvered quartz fibre. 



I constructed a very simple form of string electrometer, and 

 was testing it, when through Mr N. R. Campbell I learnt that 

 Dr C. V. Burton had designed a model of a similar instrument. 

 The Cambridge Scientific Instrument Company kindly lent me it 

 to test. The results given below were obtained with this instru- 

 ment, after it had been somewhat altered. 



The Einthoven Galvanometer. 



Einthoven has found that the movements of a stretched quartz 

 fibre subject to a varying lateral force afford a precise and delicate 

 means of measuring that stress. With his string galvanometer, in 

 which a silvered fibre replaces the usual suspended coil, he has 

 obtained photographs of the moving fibre on a falling plate using 

 a magnification of about 500. Even with this magnification the 

 motion of the fibre was so free from irregularities of an extraneous 

 origin that Einthoven has been able to subject the oscillograph 

 curves thus obtained to a minute analysis*. He has also followed 

 vibrations with a frequency of 3000 per sec. The lightness 

 (from 6 X 10~^ to 5 x 10"'' gm. per cm.) and excellent elastic 

 properties of the silvered quartz make such results possible. 



Description of the Instrument. 



The string electrometer (see fig. 1) used in these experiments 

 consisted of two vertical insulated plates, A, B, 15 cm. long by 

 •8 cm. thick with their edges parallel and at an adjustable distance 

 apart. 



* Kon. Ak. van Weten, te Amsterdam, viii. p. 210, 1906. 



