CHAPTER VIII 



INDUCTION INSTRUMENTS 



The Induction Principle. In 1826 Arago discovered that if a 

 copper disc be rotated about a vertical axis and immediately 

 below a magnetic needle which is pivoted coaxially with the 

 disc, the needle is deflected in the direction of rotation of the disc. 

 If the speed be sufficiently high the force acting on the needle will 

 overcome the earth's directive force and the needle will be set 



in rotation. The inverse experi- 

 ment may be performed; if the 

 magnet be rotated on its pivot 

 and the disc be free to move, the 

 disc will follow the magnet. The 

 correct explanation of the phenom- 

 ena was given by Faraday, who 

 showed the motion to be due to 

 the reaction between the magnet 

 and the currents induced in the 

 disc. 



In this inverse experiment, the 

 phenomena are those associated 

 with a rotating magnetic field; 

 that is, a field of constant in- 

 tensity whose angular position in 

 FIG. 256. Illustrating Ferraris' space is continually changing. In 

 method of producing a rotating 1885 Ferraris showed that a rotat- 

 magnetic field. . -1111 



ing magnetic field could be obtained 



by the action of currents in suitably placed coils. 



Let the coils be arranged as in Fig. 256, and consider the 

 magnetic field at the point 0. At any instant the field perpen- 

 dicular to the plane of the coil A at this point is given by h A = 

 k A i A where k A is a constant depending upon the geometry of the 

 coil, and i A is the instantaneous value of the current. If sinu- 



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