Damping of Galvanometer Needles. 



563 



Glasgow to the designs of Messrs. T. & A. Gray, which is 

 illustrated in figs. 11 and 12 of the paper above referred to. 

 In this instrument there are a pair of long needles arranged 

 to form an astatic combination, and each having one end 

 sucked into and the other end pushed out of a coil, the four 

 coils being enclosed in rectangular ebonite boxes. These 

 needles are attached by an aluminium bridge to a vertical 

 aluminium wire fastened at its upper end to the suspending 

 fibre, and carrying at its lower end a pair of mirrors fastened 

 back to back, the one being concave and the other plane, and 

 also carrying at its lower end a small magnetic needle between 

 a pair of auxiliary coils intended to be joined up in series with 

 a battery and reversing-key and used as damping coils. The 

 strength of the controlling field is varied by moving a magnet 

 placed on the bench on which the instrument stands up to or 

 away from the galvanometer. The results obtained showed a 

 decided steady increase in the ratio t/\ with an increase of t. 

 (See Table II. fig. 2.) 



Table II. — Ballistic Galvanometer. 



Period,*, in ) n .g 19 . g 20 . 21#1 22 . 9 23>1 3(H 45>6 5g . 2 



seconds ...S 

 Logarithmic j 

 decrement, A I 

 Ratio, t/\ 180-8 185*0 188*6 186*8 187*0 190*8 195*3 200*6 205*5 210*3 



597 

 0*0654 0*106 0*106 0*113 0*122 0*121 0*154 0*227 0*283 0*284 



Fig. 2. 



«?J_ 



Sao 



/So 



X - I 















V 



1 _ ; J '( = B- -- = = "?- 



4- ________ £_____--= ± 



__ *-,E5_ - - 















20 



30 



to 



SO 



6o 



Period, t, in Seconds. 



Finally, Mr. Taylor made some experiments in 1896 upon 

 a Rosenthal galvanometer. The suspended system of this 

 instrument is illustrated in fig. 10 of the paper previously 

 referred to, and consists of an astatic pair of needles fastened 

 below the mirror, each needle being bent into the shape of a 

 horseshoe. One pole of each needle passes into the centre 

 of a coil, whilst the other pole is on the outside of the coil, 

 and the needles are therefore sucked into or pushed out of 

 these coils by the action of the current, The results of these 

 experiments showed an increase in the ratio t/\ as t increased. 



