6 Mr. A. Campbell on the Magnetic Flaxes in 



will be discussed a little further on. To get an idea of how 

 the flux in the steel varies from point to point along the 

 annular magnet, an experiment was made with a ring-magnet 

 of rectangular section, having an air-gap as shown in fig. 2. 

 A small search-coil which could only just slip along the 

 magnet was moved by jerks into successive positions, and the 

 corresponding changes in the flux were calculated from the 

 throws on a ballistic galvanometer in circuit with the coil. 

 Fig. 2 shows the result, the radial breadth of the shaded 

 part being drawn proportional to the flux in the steel at each 

 position. Fig. 3 is a similar diagram for an ordinary bar- 

 magnet. In the ring-magnet the available air-gap flux was 

 less than one third of the maximum flux at a a. 



(5) Weston Voltmeter. — It will be noticed that B in the 

 air-gap of this instrument is very high, viz. 870. This might 

 lead one to suppose that the earth's field would have no 

 perceptible effect on its readings, but it must be remembered 

 that the flux induced in a piece of iron or steel in the earth's 

 field is usually very many times greater than the flux in air 

 due to the earth alone. This can be easily shown by connect- 

 ing a coil with a ballistic galvanometer and reversing the 

 coil with regard to the earth's field first by itself and then 

 with a soft iron core in it. The throws of the galvanometer 

 will be enormously increased by the presence of the core. 

 To find how far the earth's field affected the flux in a perma- 

 nent magnet with a moderate air-gap, a coil was wound upon 

 the circular one shown in fig. 2, and was connected with a 

 galvanometer. The magnet was then turned round so as to 

 quickly reverse the action of the earth's horizontal field upon 

 it. The resulting throw on the galvanometer showed that the 

 maximum B in the steel, which was about 5000, was only 

 changed by 3 lines per sq. cm., i. e. by less than 0*1 per cent. 

 The behaviour of the Weston magnet tallies with this, for when 

 the instrument, with a steady voltage on its terminals, was 

 turned round to face each point of the compass, no change 

 in the reading could be detected, although the scale could be 

 read to about 1 in 1000*. 



(6) Davies Voltmeter. — In this instrument one side of the 

 rectangular moving coil moves in a narrow cylindrical air- 

 gap between specially shaped pole-pieces of a strong perma- 

 nent magnet. It gives a maximum angular deflexion of 

 about 210°. 



(7) Ever shed Ammeter. — A coil of 6 turns magnetizes 

 two small pieces of iron with a movable piece between them, 



* At Professor Ayrton's suggestion I have re-tested the instrument at 

 the higher readings, and have detected a variation of about Ol per cent. 



