254 Sir David Salomons on the Electric 



In fact, all points on similar lines of force in a magnetic 

 field lying between the N. and S. poles make up a figure 

 somewhat like the shell of an egg ; and the appearance of 

 what I term the visible field appears to be like a vast num- 



Fi£. 14. 



ber of transparent coloured egg-shells placed symmetrically 

 one within the other, although, of course, each shell is not 

 symmetrical with any other, the inner ones being more elon- 

 gated and the outer ones more spherical, the centre one of 

 all being a straight line. 



By holding the tube which has been experimented with in 

 various positions in regard to the poles of the magnet, whether 

 they be N. and 8., or both N. or both S., a variety of appear- 

 ances are seen in the tube according to the position in which 

 it is held. All the effects can be traced to what I term the 

 Amperian explanation. 



From the experiments which have been described it is 

 evident that a very important opening suggests itself as to 

 the practical use of vacuum-tubes, viz. that of exploring the 

 magnetic field. I have used such tubes for this purpose for 

 many years past, and have been able to plot out in a few 

 minutes mentally that which takes a long time by the methods 

 generally employed. I am ready to admit that the usual 

 process is more accurate when absolute determinations are 

 required ; but usually all that is wished for is to ascertain the 

 general character of a field and the extent of the leakage, and 

 this can be done at once by employing vacuum-tubes. 



