624 



Prof. 0. Steels on a 



of magnetic fields— in the neighbourhood of a coil B con- 

 nected to the rings of a bipolar commutator D running 

 at a constant velocity. Close to the magnet we place an 

 electromagnet — an iron rod D surrounded by a coil A — 

 connected to a battery, the current being regulated by the 

 rheostat R and read on the ammeter A, fig. 1. 



Fiir. 1. 



M 



e 



m 



f •=■ 



If we consider either the positions of the electromagnet A 

 shown in fig. 2 or in fig. 3, we observe that the needle N is 



Fig. 2. 



Fig.2 



\**t 



Amper 



Fig. 3, 



* 



*h-HcwKt 



D 

 A 



Fig. 3 



at rest when the current I is cut off. If we increase this 

 current, we see the needle starting a periodical vibration the 

 amplitude of which rises with the magnitude of the current I, 

 passes through a maximum, and decreases till, for a sufficiently 

 large current I, the needle is again at rest. 



