WHITWORTH'S MICROMETRIC APPARATUS. 



formed, partly by friction against each other, and partly by 

 abrasion with a peculiar tool. 



So plane are the surfaces of metal thus formed, that when one 

 is laid upon the other no one part comes into closer contact than 

 another, and there is included between them a stratum of particles 

 of air which act like infinitely smooth rollers, and the surfaces 

 move in contact one with another with a degree of freedom, owing 

 to the lubricity of the air, which must be felt to be conceived. 

 If, however, the surfaces be so severely pressed against each other 

 as to exclude the air, the contact becomes so complete that it is 

 with great difficulty they can be separated. 



These surfaces, thus accurately formed, are used as standards 

 to test other plane surfaces, and with these are tested the ends of 

 a standard measure of metal, which is placed in an accurately 

 formed horizontal metallic bed. One end bears against a metallic 

 pin ; another metallic pin, urged by a screw, presses against the 

 other end ; and if this metallic bar, by change of temperature or 

 any other cause, suffer a change in its length amounting to the 

 millionth part of an inch, that change is rendered perceptible by 

 the following arrangement ; 



The pin which bears against its extremity is moved by a screw, 

 which has ten threads to the inch. On the head of this screw is 

 a wheel, consisting of 400 teeth, which works in a worm driven 

 by another wheel, the rim of which is divided into 250 visible 

 parts. Now, since each thread of the original screw corresponds 

 to the l-10th part of an inch, each tooth of the wheel upon its 

 head will correspond to the 4000th part of an inch, and each divi- 

 sion of the wheel attached to the worm will correspond to the 

 millionth part of an inch. 



It is found, in the application of this apparatus, that a change 

 in the position of the wheel attached to the worm, through one of 

 the 250 divisions, is rendered sensible at the point of the screw 

 which bears against the standard bar ; but since the motion of the 

 former wheel through one division can produce a motion amounting 

 only to the millionth part of an inch in the point of the screw, this 

 magnitude is thus rendered sensible. 



To prove the accuracy of this micrometric apparatus, a 

 standard yard measure made of a bar of steel, about three 

 quarters of an inch square, having both the ends rendered 

 perfectly true, was placed in it. One end of the bar was then 

 placed in contact with the face of the machine, and at the other 

 end, between it and the other face of the machine, was interposed 

 u small flat piece of steel, termed by the experimenter "the 

 contact piece," whose sides were also rendered perfectly true and 

 parallel. Each division on the micrometer represented the one- 



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