334 THE PRINCIPLES OF SCIENCE. 



meter to a standard current r . In short, lie measures not 

 the current itself but a known fraction of it. 



In many electrical and other experiments, we wish to 

 measure the movements of a needle or other body, which 

 are not only very slight in themselves, but the manifes- 

 tations of exceedingly small forces. We cannot even 

 approach a delicately balanced needle without disturbing 

 it. Under these circumstances the only mode of proceed- 

 ing with accuracy, is to attach a veiy small mirror to the 

 moving body, and employ a ray of light reflected from 

 the mirror as an index of its movements. The ray may 

 be considered quite incapable of affecting the body, and 

 yet by allowing the ray to pass to a sufficient distance, 

 the motions of the mirror may be increased to almost any 

 extent. A ray of light is in fact a perfectly weightless 

 finger or index of indefinite length, with the additional 

 advantage that the angular deviation is by the law of 

 reflection double that of the mirror. This method, was 

 introduced by Gauss, and is now of great importance ; 

 but in Wollaston's reflecting goniometer a ray of light 

 had previously been employed as an index finger. Lavoi- 

 sier and Laplace had also used a telescope in connection 

 with the pyrometer. 



It is a great advantage in some instruments that they 

 can be readily made to manifest a phenomenon in a greater 

 or less degree, by a very slight change hi the construction. 

 Thus either by enlarging the bulb or contracting the tube 

 of the thermometer, we can make it give more conspicuous 

 indications of change of temperature. The barometer, on 

 the other hand, always gives the variations of pressure 

 on one scale. The torsion balance is especially remark- 

 able for the extreme delicacy which may be attained 

 by increasing the length and lightness of the rod, and the 



r De la Rive's ' Electricity,' vol. ii. pp. 897, 98. 



