476 Intelligence and Miscellaneous Articles. 



ON A NEW FORM OF GALVANOMETER FOR POWERFUL CURRENTS. 

 BY TROFESSOR C. F. BRACKETT, COLLEGE OF NEW JERSEY. 



The very powerful currents produced by large dynaino-ruachines 

 are not easily estimated by the appliances usually found in the 

 physical laboratory. 



The various forms of the electro -dynamometer, the cosine-galva- 

 nometer, and some other special forms of apparatus may of course 

 be employed with satisfactory results. None of these instruments 

 beiug in the collection at Princeton, it was determined to construct 

 a galvanometer which should obey the law of tangents and yet not 

 be so large as to be unwieldy. In order to do this, recourse was 

 had to the differential principle. The construction is as follows : — 



Two stout hoops of copper or brass of different diameters are 

 very exactly turned in the lathe. They are then each cut open at 

 one point and joined to each other concentrically, by soldering with 

 hard solder, between the ends on one side of the cut, a piece of 

 metal having the same cross section as that of the hoops, and of 

 suitable length. At several other points are inserted between the 

 hoops pieces of hard rubber of proper thickness, which serve to 

 keep them truly concentric. 



The free ends of the system thus arranged, and set upright on a 

 proper base-board, are joined to binding- screws. 



It will be seen that the differential action on a needle placed at 

 the centre, or on the axis of the hoops pasing through their centre, 

 depends on the different distances of two equal and opposite cur- 

 rents. It is evident also that the instrument may be used as a 

 simple tangent-galvanometer. 



Thus, if we call the free ends of the hoops A and B respectiA'ely, 

 and the point of juncture C, by joining up a circuit through A and 

 B we get the differential action ; but by joining up through C and 

 A or B, the action is that of a simple tangent-galvanometer. If r 

 and r represent the radii of the outer and inner hoops respectively, 

 the ordinary formula becomes 



I = .—^ — _ x H tan d 



Z-rr (r—r) 



when the instrument is used differentially. 



An instrument has been constructed at the J. C. Green School 

 of (Science for its physical laboratory, of dimensions as follows : — 



Diameter of outer hoop 10*9 centim. 



Diameter of inner hoop 9*96 „ 



Width of each hoop 2-3 „ 



Thickness of each hoop 0*35 ,, 



The theoretical constant of this instrument agrees very closely with 

 that ascertained experimentally by means of the voltameter. The 

 needle, which is usually suspended in the centre of the hoops, may, 

 if desired, in order to measure exceedingly powerful currents, be 

 moved along their axis to any required distance on one side. The 

 constant is in that case easily found by an obvious modification of 

 the formula. — Silliman's Amer ican Journcd, May 1881. 

 Princeton, March 10, 1881. 



