562 



Mr. M. Solomon on the 



mirror affixed at the centre. The mirror is enclosed in a 

 small case with glass back and front so arranged that these 

 glasses can be screwed nearer together or farther apart, thus 

 enabling the space in which the mirror swings, and conse- 

 quently the air damping, to be varied. A pair of coils is 

 placed one behind and one before the upper set of magnets, 

 and a similar pair before and behind the lower set, the coils 

 being enclosed in ebonite boxes fitted into a brass frame. By 

 thus placing the mirror outside, instead of at the centre of 

 the coils, an improvement introduced into the Thomson 

 galvanometer by Mr. Mudford, the coils may be wound 

 practically to the centre and much valuable space utilized. The 

 strength of the controlling field is varied by sliding a magnet 

 up and down a vertical upright fixed to the top of the instru- 

 ment case. As a result of these experiments Mr. Seaman 

 found that the ratio t/\ increased as the period increased, 

 becoming, however, nearly constant at long periods. The 

 actual results are given in Table I., and plotted as a curve in 

 fig. 1. This curve is plotted with the values of £, the observed 

 period, as abscissae, and the values of t/K as ordinates, the 

 values of t/X being taken in preference to those of \/t, as 

 might appear more rational, because, as has been shown 

 above, it is the ratio t]\ that occurs in the expression for the 

 time of the reduction of the deflexion to 1/m of its original 

 value. 



Table 1. — Mudford's Thomson Galvanometer. 



Period, t, in seconds 3 75 



Logarithmic decrement, X 00631 

 Ratio,*/A 59-4 



4-63 

 00734 

 632 



5-46 



0854 

 645 



724 1106 

 0-1112 0-1725 

 65-1 64-2 



1455 

 02268 

 641 



Fig. 1 



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t§ 60 _:£:£_ _ t 





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o as j- r 



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Period, t, in Seconds. 



A series of experiments was made by Mr. Macquay[in 1896 

 on a ballistic galvanometer, made by Messrs. White of 



