an Electric Micrometer. 545 



In the earlier observations, when the distance was required 

 between any two positions, they were both read and entered 

 (say for instance 48*92--49 , 43, diff. = 051), but subsequently 

 readings were entered always as differences ; this method was 

 less tedious and conducive probably to greater accuracy. 



II. Observations and Results. 



Passing now to the experimental results we have them 

 arranged as follows : — 



(1) Preliminary. 



(2) Calibration of the instrument. 



(3) Measurement of the diaphragm movement throughout 

 the range of sound impulses, from the smallest audible 

 ones to such as are very loud. 



(4) The damping of the diaphragm. 



(1) Preliminary. 



It was necessary at the outset to ascertain whether this 

 contact method was delicate and consistent, and if so under 

 what conditions. Several months were occupied in endeavours 

 to obtain consistent readings of the contact position of the 

 diaphragm ; detail after detail was improved in the apparatus 

 as its requirements were discovered. Three tables will be 

 given as samples of the readings taken. 



Table I. explains itself (p. 546). 



Table II. shows how the displacement of the diaphragm due 

 to a steady current is taken. Column 2 gives the diaphragm 

 position before, and column 4 after current is put on. Column 

 3 is found by assuming the creep in contact uniform. 

 Column 7 shows the movement in terms of the unit yu/4 (10 -7 

 centim.). This table was obtained in earlier work, and it is 

 not specially accurate, but it serves to show that the readings 

 are very fairly consistent. 



Table III. Here a measurement was made of the throw of 

 the diaphragm ; the electromotive force and resistance used 

 are known, but, on account of self-induction in the receiver, 

 the current does not rise at once to its full amount. It is this 

 uncertainty about the action of self-induction which has 

 precluded the use of the throw in subsequent work. 



