2O2 THE SCIENTIFIC PAPERS OF 



tion 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 position 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 the drawing, the contact pieces are all separate, and the 

 current has to pass through the entire series of coils, which 

 present sufficient aggregate resistance to prevent the current from 

 exceeding the desired limit. 



When the minimum current is passing, the thin metallic strip is 

 at its minimum working temperature, and all the metallic prisms 

 are in contact, this being the position of least resistance. As soon 

 as the current passing through the apparatus shall increase in 

 amount, the thin metallic strip will immediately rise in tempera- 

 ture, which will cause it to elongate, and will allow the lever (L) 

 to recede from its extreme position, liberating one contact piece 

 after another. Each such liberation will call into action the 

 resistance coil connecting the spring ends, and an immediate 

 corresponding diminution of the current through increased resist- 

 ance ; additional resistance will thus be thrown into the circuit, 

 until an equilibrium is established between the heating effect 

 produced by the current in the sensitive strip, and the diminution 

 of heat by radiation from the strip to surrounding objects. In 

 order to obtain uniform results, it is clearly necessary that the loss 



