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 SCIENCE AND PRACTICE. 251 



directive force. The instrument so placed has its maximum 

 sensibility. 



The scale, divided from the middle towards the ends into 

 equal parts, is fixed upon a wooden stand at a distance of 

 two or three feet from the mirror. Behind the scale is a 

 parafiin lamp, whose light falls through an adjustable slit 

 underneath on to the mirror, which reflects it back upon 

 the scale ; and in order that the point of light shall be 

 as well defined as possible, a small plano-convex lens is 

 placed before the mirror, through which the rays converge 

 into a focus, throwing a sharp image of the slit upon the 

 scale. 



Mr. Yarley has made some of these instruments for the 

 measurements of the Atlantic cable, and has substituted a 

 plano-convex lens, silvered on the curved side, for the mirror 

 in Professor Thomson's instrument, dispensing of course with 

 the lens in front. 



23. Rheostats. In the early experimental investigation of 

 the laws of the galvanic current, the comparison of resist- 

 ances was made by lengths of metal wire, which becoming 

 sometimes rather great, an inconvenience was very soon 

 felt in handling them. Wheatstone first overcame this 

 by rolling the wire round a cylinder of dry boxwood, on 

 which a worm was cut just deep enough to receive it 

 comfortably, and to facilitate the variation of its length ; 

 the other end of the wire was coiled upon a cylinder of brass 

 in such a way that the point where the wire touched the 

 cylinder as a tangent to its circumference should be the 

 point of contact, and from this point the length of the wire 

 on the non-conducting roller to the end was measured. 

 The cylinders of boxwood and brass were fixed in bearings 

 parallel to each other upon a wooden board. The worm on 

 the wooden roller was cut from end to end, comprising about 

 forty turns to the inch. The wire, whose thickness did not 

 exceed the one-hundredth of an inch, was connected at one end 

 to a metal cap which covered the nearer end of the wooden 

 roller, round which it followed the course of the worm until 

 it left it to be wound upon the metal cylinder, to the further 



