406 



OPTICAL PROJECTION 



FIG. 227 



in shadow. On switching off the current the armature will 

 be seen still attached, with even a considerable load ; but on 

 striking the core sharply with a small hammer so as to re- 

 lease the molecular 

 rigidity, it at once 

 falls. 



The attractive 

 pull of a solenoid 

 upon a magnetic 

 core is an important 

 phenomenon, being 

 so widely employed 

 to regulate arc- 

 lamps. It is very 

 easily projected, fig. 

 228 showing one of Prof. Forbes's arrangements. The solenoid 

 A, here in conventional connection with a battery, is arranged 

 across one half of the condenser ; and the soft iron core B is 



suspended by threads 

 so as barely to enter 

 one end. In order 

 that the iron core may 

 be able to enter far 

 enough, it is well to 

 prolong it by a non- 

 magnetic wire brazed 

 to one end, so that the 

 whole may balance 

 by the threads ; or the 

 core may be supported 

 in the middle of a 

 much longer glass 



tube, whose ends are suspended by threads attached far outside 

 the solenoid, so as to give ample motion. Directly the current 

 is switched on, the core is drawn into the coil. A still simpler 



