On the Damping of Galvanometer Needles. 559 



The sources of voltaic current employed varied from a 

 single couple to four hundred in series of small copper and 

 zinc plates in a slightly acidified solution of sodic sulphate, 

 the galvanometer being used differentially and the current 

 divided in parallel between the coil in the lead and that in 

 the paraffin-box. With the four hundred cells a considerable 

 current passed. 



The greatest care was taken to adjust the galvanometer to 

 perfect neutrality with the full current passing, and to make 

 each pair of coils perfectly equal in resistance when placed 

 together in the centre of the space between the two small 

 boxes and at exactly the same temperature. 



After very numerous experiments extending over many 

 months ; frequently exchanging the situations of the coils in 

 the cube of lead and the paraffin-box ; making corrections 

 for minute differences in the two thermometers and small 

 changes in the galvanometer ; and in all cases taking the 

 observations of conduction-resistance when the two coils were 

 at exactly the same temperature in the two boxes, no 

 perceptible difference could be detected in the resistance of 

 either of the pair of coils by the influence of the lead : the 

 influence of the lead, therefore, if any, must have been 

 excessively minute. A single millimetre of movement of 

 the reflected image upon the scale at a distance of one metre 

 from the needles could easily be detected ; and the current 

 from a single cell of the battery was quite sufficient to send 

 the image rapidly off the scale when the galvanometer was 

 used nondifferentially. 



LVII. On the Damping of Galvanometer Needles. 

 By Maurice Solomon*. 



IT is well known that shortening the period of oscillation of 

 a galvanometer-needle by increasing the strength of the 

 magnetic controlling field decreases the decrement, or ratio of 

 one complete swing to the next. It follows therefore that for 

 a given initial amplitude of vibration a needle swinging in a 

 strong controlling field will make a greater number of 

 oscillations before coming to rest than when swinging in a 

 weak field ; but since the time of each oscillation is less in 

 the former case it does not follow that tbe time required for 



the amplitude to be reduced to a given fraction (say — ) of 



its initial value is greater with a strong than with a weak 

 controlling field. 



* Communicated bv the Physical Society: read March 9, 1900. 



