1879.] Measuring and Regulating Electric Currents. 293 



On the 19th of June last, upon the occasion of the Soiree of the 

 President of the Royal Society, I exhibited a first conception of an 

 arrangement for regulating such currents, which I have since worked 

 out into a practical form. At the same time, I have been able to 

 realize a method by which currents passing through a circuit, or 

 branch circuit, are measured, and graphically recorded. 



It is well known tha,t when an electric current passes through a 

 conductor, heat is generated, which, according to Joule, is propor- 

 tionate in amount to the resistance of the conductor, and to the 

 square of the current which passes through it in a unit of time, or 



H = C 2 R. 



I propose to take advantage of this well-established law of electro- 

 dynamics, in order to limit and determine the amount of current 

 passing through a circuit, and the apparatus I employ for this pur- 

 pose is represented on figs. 1 to 3, Plate 4, of the accompanying 

 drawings. Letters of reference to the principal parts of the instru- 

 ment are given on the foot-note of the drawing. 



The most essential part of the instrument is a strip (A) of copper, 

 iron, or other metal, rolled extremely thin, through which the cur- 

 rent to be regulated has to pass. One end of this thin strip of metal 

 is attached to a screw (B), by which its tension can be regulated ; it then 

 passes upwards over an elevated insulated pulley (I), and down again to 

 the end of a short lever, working on an axis, armed with a counter- 

 weight and with a lever (L), whose angular position will be mate- 

 rially affected by any small elongation of the strip that may take 

 place from any cause. The apparatus further consists of a number of 

 prisms of metal (P), supported by means of metallic springs (M), so 

 regulated by movable weights (W) as to .insure the equidistant posi- 

 tion of each prism from its neighbour, unless pressed against the 

 neighbouring piece by the action of the lever (L), in consequence of a 

 shortening of the metallic strip. By this action, one prism after 

 another would be brought into contact with its neighbour, until the 

 last prism in the series would be pressed against the contact spring 

 (S), which is in metallic connexion with the terminal (T). 



The current passing through the thin strip of metal will, under 

 these circumstances, pass through the lever (L) and the line of prisms 

 to the terminal (T), without encountering any sensible resistance. A 

 second and more circuitous route is, however, provided between the 

 lever (L) and the terminal (T), consisting of a series of comparatively 

 thin coils of wire of German silver or other resisting metal (R, R), 

 connecting the alternate ends of each two adjoining springs, the first 

 and last spring being also connected to the lever (L) and terminal (T) 

 respectively. 



When the lever (L) stands in its one extreme position, as shown in 



