562 On an Electric Micrometer. 



On this basis plot a curve for the impulse (fig. 6), 



Fig. 6. 



a, 



From ten observations on small distances the mean value 

 for p was approximately §. 



Besides the work above indicated, many other measure- 

 ments were made, viz. (1) microphone action of various 

 substances ; (2) the movement of the receiver-diaphragm 

 when in circuit with a transmitter in action; (3) apparent 

 coherence of the surfaces under certain conditions. 



These applications will be reserved at present. 



Summary. 



The main purpose of this paper is to show : — 



(1) That this method of measuring small distances by 



electric contact is reliable. 



(2) That, whereas in other methods for measuring small 



distances a limit to working is made by the size of 

 some wave-length, in this method the limit is far 

 lower, is not thus dependent, and has not been reached 

 in the apparatus so far made. 



(3) That a measurement can be made of the smallest audible 



impulsive sound given by a telephone-diaphragm. 



(4) That the method will probably have many other 



applications, where small measurements are involved. 



The author wishes to thank Mr, W. H. Everett, B.A.,B.E., 

 for some valuable suggestions, arid Professor W. H. Heaton, 

 M.A., for his kind interest in this research. 



