the Ultra-Micrometer. 635 



For a sensitive indicator 



d R= ] 



dx 2(7rLA.r)^ 



should be as large as possible, that is, the product LA.i' 



should be small. ^ 



In order to get some idea of the actual value of ~f to be 



dx 



expected under practical conditions we may conveniently 



rewrite the above in the form 



dx 



N 



A value of N easily obtained under conditions shortly to b« 

 described is 10 6 ; assuming #== To J oq inch, say, we have 



cm 



dx 



10 9 



By a beat method — explained later — it is possible to 

 observe a change in N of less than unity — assuming &N=1 

 we get the corresponding value of Sx = 2 X 10 9 inch, 

 = 5xl0~ 9 cm. 



With this introduction the following description of an 

 apparatus built along such lines is put forward. 



Fig. 1 shows the apparatus in diagram. A is an oscil- 

 lating valve circuit involving the parallel-plate condenser (P) 

 discussed above; T is a loud-speaking telephone shown for 



Fiff. I. 



B 



A 



li'Kf 



4b£ 



r~\ 



'I'HVWWWI/li 



I'NWWV 



a 



simplicity directly inserted in the valve anode circuit, 

 although in actual practice a three-stage transformer am- 

 plifier intervened, to magnify suitably the currents passing 

 through 1. 



2 T 2 



