Galvanometers, and on the Rigidity of Silk Fibre. 51 



end of a U-tube, containing phosphoric acid and beads of 

 glass, the other end of which was sealed to a tube leading to 

 the Sprengel pump. The vibrator was attached to the fibre 

 by means of three short single fibres, in the manner shown in 



the sketch. The results are given in the following table, the 

 meaning of the numbers in the different columns of which 

 will be readily understood from the headings. In the column 

 headed " numbers of vibrations observed " the figures repre- 

 sent roughly the number of periods which could be observed 

 at the different degrees of exhaustion, shown in the preceding 

 column, beginning in each case from an amplitude of about 

 60°, and observing directly the transits of a black spot on the 

 ring over a fixed mark until the amplitude fell to about 10°. 



The results are sufficient to show that the effect of the 

 viscosity of the fibre in damping the vibrations is very small 

 in comparison with the effect of the air friction; and it seems 

 probable that amoderatelyheavyvibrator(sayabout 2 grammes 

 in weight) with a small magnetic needle attached, and sus- 

 pended by a single silk fibre, may prove a good arrangement 

 for experiments such as have been carried out by Maxwell, 

 Kundt and Warburg, Crookes and others on the friction and 

 viscosity of gases. It certainly would have the advantage 

 that the period would depend mainly on the strength of the 

 magnetic field, and could be varied at pleasure. Should 

 opportunity offer, Mr. Bottomley and the writer hope to con- 

 tinue these experiments. 



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