Electrical Method for Measurements. By P. E. Shaw. 629 



taken in any measurement made. Each step involves errors and 

 expenditure of time. 



On the other hand, the electric micrometer is a direct method ; 

 only one measurement is required, thus making for accuracy, 

 whilst at the same time the instrument used is very sensitive. 



If one specially wishes to work by means of an eye-piece 

 micrometer, it is easy to see how the electric method can be used 

 in place of the stage micrometer to give a very accurate calibra- 

 tion of the eye-piece. But this does not seem a desirable course. 



(4) Some modern Microscope stages are moved by a screw 

 which has a graduated head. In this way a measurement may be 

 taken directly (as with the electrical method). In principle this 

 micrometer is good, but in practice it has three disqualifications : — 



Fig. 128. 



(i.) The measuring-screw is given the work of carrying the stage 

 with it when it turns ; supposing no looseness or backlash occur 

 at first, these will probably be set up in time from the wearing- 

 action, and so errors will arise, (ii.) It is inconvenient to have 

 such a large graduated disc attached near the stage as will give 

 readings of 1 micron ; but the disc is generally small, and hence 

 the readings cannot be very fine or the instrument delicate, (in.) 

 The micrometer screw being fixed on the stage will ' be liable to be 

 knocked and damaged, and so spoiled for accurate measurement. 

 Any fine measuring-screw should be put away when not in use. 



The Electric Micrometer is like the above in being direct- 

 acting, but it does not suffer from the disqualifications mentioned. 

 It can be used with any Microscope having a movable stage (and 



