an Electric Micrometer. 



543 



It would seem desirable to produce the highest possible 

 polish, for doubtless the irregularities on the surfaces, even 

 under the best conditions, are large compared with the smallest 

 measurements to be made. The ideal surfaces would be a 

 geometrical sphere and geometrical plane, whereas the actual 

 contacts would, if very closely examined, appear in point of 

 roughness more like mountainous regions. 



5. Circuits. 



The simple circuit, shown in plan in fig. 1, was superseded 

 by one shown (also in plan) in fig. 2, where we see the 

 observiug-receiver (Rec) and a condenser (S) put together 

 as a shunt to the large resistance (Rj. Another, small, 

 resistance (R 2 ) is used, and the cell (C) can be reversed. 



Fur. 2. 



If the circuit is completed by the touching of the contacts 

 (a, b), there is (1) a momentary rush of electricity to charge the 

 condenser, (2) a steady current of small amount through the 

 resistances R t and R 2 . (1) soon ceases. On " breaking " 

 circuit, by the separation of a and b, the condenser will be 

 discharged through R 1? and so, at the next "make," will receive 

 a new charge, and so on. 



Thus at both " make " and " break " a rush occurs through 

 the telephone (Rec), giving the observer evidence of them ; 

 the rush lasts for a very short time, but it can be made 

 powerful enough to serve our purpose. 



By the use of a condenser we obtain a large impulsive flow 

 just when required, i. e. at " make " and " break ;" hence the 

 contact is not disturbed by a steady flow (and consequent 

 heating &c.) during the interval between make and break. 



