354 Dr. C. W. Siemens on the Transmission and 



preference), which by its expansion and contraction regulates 

 the current passing through it. This strip is rolled down to 

 a thickness not exceeding O05 millim., and is of such a 

 breadth that the current intended to be passed through the 

 regulated branch circuit would raise the temperature of the 

 strip to say 50° C. 



This strip of metal (A) is stretched horizontally between a 

 fixed support and a regulating-screw (B), at which latter the 

 current enters, passing through the strip, and thence through 

 a coil of German-silver wire (C) laid in the form of a collar 

 round the centre, and connected at its other extremity with a 

 binding-screw (D), whence the current flows on towards the 

 lights or other apparatus to be worked by electricity. Upon 

 its middle the strip carries a saddle of insulating material, 

 such as ebonite, upon which rests a vertical spindle, support- 

 ing a circular metallic disk (E), with platinum contacts 

 arranged on its upper surface. Ten or any other number of 

 short stout wires connect the helical rheostat at equidistant 

 points with adjustable contact-screws (F), standing above the 

 platinum contacts on the surface of the metallic disk. These 

 wires are supported upon the circular frame (G) of wood or 

 other insulating material, but are free to be lifted off their 

 support if the metallic disk should rise sufficiently to be brought 

 into contact with the screws. These latter are so adjusted 

 that none of them touches the metallic disk when it is in its 

 lowest position, but that they are brought one after another 

 into contact with the same as the disk rises ; and it will be 

 easily seen that for every additional contact-screw that is 

 raised seriatim by the disk, a section of the helical rheostat 

 between attachment and attachment is short-circuited by the 

 metallic disk, and thus excluded from the circuit. When the 

 disk is in its uppermost position the whole of the rheostat is 

 short-circuited, and the regulator offers no other resistance to 

 the current than that of the horizontal strip itself. In setting 

 the regulator to work the regulating-screw (B) is drawn on 

 sufficiently to bring the whole of the contact- screws into con- 

 tact with the disk. The passage of the current through the 

 strip will have the effect of raising its temperature to an ex- 

 tent commensurate with the electrical resistance; and in the 

 same measure the strip itself will be elongated, and cause the 

 spindle with the contact-disk to descend. 



Another form of this instrument depends for its action 

 upon the circumstance discovered by the Count du Moncel in 

 1856, and more recently taken advantage of by Mr. Edison, 

 that the electrical resistance of carbon varies inversely with 

 the pressure to which it is subjected. A steel wire of 0'3 



