A POCKET GALVANOMETEE. 



299 



into the back corner of the case of the instrament. The connection 

 of the terminals is diagramatically shown in Fig. 11. 



For the measurement of potential diiferences, the terminals ( 1 ) 

 and (3) are used and for the measurement of small currents the 

 terminals (1) and (2). To avoid the possibility of confusion as to 

 which pair of terminals is to be used,' a pair of binding screws are 

 provided at (1) and (2) for the case of small currents, and a pair of 

 wire ropes at (1) and (3) for the case of potential differences. An 

 alternative construction would be to omit the pair of binding screws 

 and provide a plug hole between (2) and (3) to shunt off the re- 

 sistance when the instrument is to be used for small currents. For 

 the measurement of very large potential differences or of moderate 

 currents a system of shunts may be employed in tlie usual way. 

 A-^arious other like devices may be multiplied almost endlessly. 



A separate calibration is required for this coil, but this is 

 nothing more than the ordinary galvanometer gauging. The follow- 

 ing table gives a comparison of the readings of an actual instrument 

 with those of a standard sfalvanometer. 



Staiulard Galvanometer. 



Pocket Galvanometer. 



Eeadinfc. 



Keduced to 

 Amperes. 



Keading. 



Value of 1 Division 

 in Amperes. 



7.8 

 ]3.G 

 22.1 



.02278 

 .03971 

 .06453 



25.8 

 45.1 

 73.2 



.000883 

 .000880 

 .000882 



Mean .000882 



From the above result we easily find, f)r the given extra 

 resistance of 5000 ohms inserted, the value of the potential dif- 

 ference corresponding to 1 division of the scale. It is 4.4 volts 

 (= .000882 X 5040). If a thinner wire be wound with a greater 



