Art. IV. — On a Form of Tangent Galvanometer suitable 

 for Measuring Powerful Currents. 



By F. J. Pirani, M.A. 



[Contributed 19th May, 1881.] 



An instrument for measuring the strength of such power- 

 ful currents as those employed in electric lighting, which 

 shall combine portability, simplicity of construction, and 

 accuracy, seems still to be a desideratum. The galvano- 

 meter, a rough model of which I exhibit this evening, 

 combines these advantages better than any instrument 

 of which I have seen a description. It consists of a 

 band of copper, bent so as to form two concentric circles, 

 which will be traversed by the current in opposite directions, 

 as shown in the accompanying sketch. At the common 

 centre of the circles is supported a magnetic needle, to which 

 is attached an index moving over a graduated circle, or a 

 mirror to reflect a beam of light on to a scale in the ordinary 

 manner. The leading wires are twisted together to prevent 

 their having any sensible influence on the needle. 



(See Diagram.) 



