72 



SEC. 3. MEASUREMENT. 



to the two positions of the roller are m 1 and 

 from equation (I.), 



and their difference is equal /, 



therefore : 



a, a. 



(II.) 



When the constant c for both rollers is calculated, the constant difference 

 in 'the results for m is fixed by a number of revolutions of the disc M in the 

 same position of the rollers. The difference of m found in this way, of course, 

 is constant. 



The equations (I.) for the apparatus exhibited, calculated as described, are : 



For the roller on the left side : 



m = 8656 -632 a2 ~ ai meters. 

 For the roller on the right side : 



m = 8655* 983 -^^ meters. 



The roller on the right side gives a constant difference for the value m 

 given with the roller on the left side equal 1'3252 meter. Respecting the 

 correction of the apparatus, it is only necessary to make the axis of the rollers 

 parallel with the direction of the movement on the disc M. In this parallel 

 position this movement, of course, has no influence on the revolution of the 

 rollers. The parallel position of the axis of the rollers may be tried by ex- 

 periment, by moving them on the disc M, turning the disc C. During this 

 experiment the disc M must remain at rest. No motion of the rollers will 

 then be observed. It seems requisite, at first sight, that the roller J move 

 through the centre of the disc M. But this is not necessary, an approxima- 

 tion only being needed for practical purposes. By a sidewards position no 

 difference in the revolution of the rollers is effected. This is proved in the 

 following way : 



Fig. 4. 



