WHITWORTH'S MICROMETRIC APPARATUS. 



millionth part of an inch, and each time the micrometer was moved 

 only one division forward, the experimenter raised the contact 

 piece, allowing it to descend across the end of the bar by its own 

 gravity only. This was repeated until the closer approximation 

 of the surfaces prevented the contact piece from descending, when 

 the measure was completed, and the number on the micrometer 

 represented the dead length of the standard bar to the one- 

 millionth part of an inch. 



Eight repetitions of the experiment in a quarter of an hour 

 produced identical results, there not being in any case a variation 

 of one-millionth of an inch. 



This method of operating was termed " the system of proof by 

 the contact of perfectly true surfaces and gravity ;" and in con- 

 nexion with it was shown another interesting experiment. 



When the micrometer was screwed up within one division of the 

 number where contact would be presumed to occur, the warmth of 

 the finger applied to the centre of the steel bar sufficed to expand 

 and lengthen it instantaneously, so as to prevent the descent of the 

 " contact piece." 



The other method of proof was by having a small simple battery 

 composed of a piece of zinc soldered on to a piece of copper and 

 plunged into rain water, without the admixture of any acid ; this 

 was connected with the two ends of the measuring machine, and 

 also with a delicate galvanometer. On pursuing the same process 

 of advancing the micrometer one division at a time, no effect was 

 produced until the last millionth of an inch of distance was 

 traversed, and absolute contact occurred with the end of the bar, 

 when the deflection of the needle of the galvanometer instantly 

 betrayed the movement. Repeated experiments showed this to be 

 unerring in the result, and on placing the finger on the middle of 

 the bar, under the same circumstances as in the other course of 

 experiments, the expansion was instantly detected by the deflection 

 of the galvanometric bar. 



By the application of this instrument, standard gauges for 

 axles, taps, and other parts of machinery which it is desirable to 

 maintain uniform, are constructed, and have been adopted by the 

 Admiralty. 



192 



