226 



MEASUREMENT OF CURRENTS. 



With a somewhat thick copper sphere, without a break, and 

 a powerfully magnetised needle, an aperiodic movement is easily 

 obtained (685). 



This arrangement, however, is not suited for sensitive galvano- 

 meters, for the introduction of a heavy mass of metal about the 

 needle increases the central cavity of the coil, the effect of which is 

 to diminish the sensitiveness. 



Having stated these general principles, we will proceed to pass 

 under review the principal types of galvanometers in ordinary use. 



847. NOBILI'S GALVANOMETER. The galvanometer which has 

 been for a long time in most extended use is that of Nobili.* The 



Fig. 162. 



needles, which form an astatic system, are portions of sewing needles, 

 5 cm. to 6 cm. in length; the coil is rectangular (Fig. 162), and the 

 frame is just large enough .to allow the fr.ee play of one of the 

 needles ; its breadth is 4 cm. to 5 cm. The lower needle is in the 

 interior of the frame ; the other is outside, and is provided with a 

 drawn-out index. The system is attached to a fibre of silk L, and 

 may be raised or lowered by means of the screw K. 



* NOBILI. Descrizione d'un Nuovo Galvanometro. Maggio, 1825. Memorie 

 ed Osseruazioni, etc., Vol. I., p. I. 



