290 ELEMENTARY LESSONS ON [CHAP. v. 



shape of its poles, and on the form of the soft iron 

 armature which it attracts. It should be so arranged 

 that as many lines of force as possible should run through 

 the armature, and the armature itself should contain a 

 sufficient mass of iron. Joule designed a powerful electro- 

 magnet, capable of supporting over a ton. The maximum 

 attraction he could produce between an electromagnet and 

 its armature was 200 Ibs. per square inch, or about 

 13,800,000 dynes per square centimetre. It can be 

 shown that the attraction of an armature of soft iron is 

 proportional to the square of the " magnetic strength " of 

 the electromagnet; for, suppose an electromagnet to have 

 its strength doubled, it will induce the opposite kind of 

 magnetisation twice as strongly as before in the iron 

 armature, and the resulting force (which is proportional 

 to the product of the two strengths) will be four times as 

 great as at first. 



LESSON XXVII. Electrodynamics. 



331. Electrodynamics. In 1821, almost immedi- 

 ately after Oerstedt's discovery of the action of a current 

 on a magnet, Ampere discovered that a current acts 

 upon another current, attracting it 1 or repelling it 

 according to certain definite laws. These actions he 

 investigated by experiment, and from the experiments 

 he built up a theory of the force exerted by one current 

 on another. That part of the science which is con- 

 cerned with the force which one current exerts upon 

 another he termed Electrodynamics. 



332. Laws of Parallel and Oblique Circuits. 

 The following are the laws discovered by Ampere : 



1 It would be more correct to speak of the force as acting on conductors 

 carrying currents, than as acting on the currents themselves. It is disputed 

 whether the current in the conductor is attracted ; we know only with 

 certainty that the conductor itself experiences a force. See, however 

 Art - 337- 



