ELECTROMAGNETISM 



ELEC'TROMAG'NETISM, the branch of 

 physical science which treats of the relations 

 between electricity and magnetism. When- 

 ever an electric current flows through a wire 

 there is a magnetic field around the wire. 

 An electric current is electricity in motion; 

 electricity in motion is always accompanied 

 by magnetic force. 



A magnetic compass placed near a wire 

 through which an electric current is flowing 

 tends to place itself perpendicular to the wire. 

 If a number of magnetic compasses are placed 

 around a wire through which an electric cur- 

 rent is flowing, the compass needles arrange 

 themselves in the form of a circle, showing 

 that the lines of force are circles. If the di- 

 rection of the current is reversed, the compass 

 needles will all turn around and again point 

 out a circle, each needle pointing in the oppo- 

 site direction from the first. Reversing the 

 current reverses the direction of the force. 



A coil with an electric current flowing 

 through it acts like a magnet. The coil has 

 a north and a south magnetic pole, as has a 

 bar magnet; it retains its magnetic force only 

 so long as the current flows. If the direction 

 of the current is reversed the polarity of the 

 coil is reversed. If an iron core is placed in 

 the coil the magnetic strength is increased (see 

 ELECTROMAGNET) . 



If an alternating current flows through a 

 wire the magnetic field is reversed every time 

 the current is reversed, as the experiment with 

 the compasses referred to above shows. In 

 this respect the magnetic field about an alter- 

 nating current differs from that of a direct 

 current, for a direct current of constant strength 

 produces an unchanging magnetic field. If an 

 alternating current flows through a coil, the 

 magnetic poles of the coil change with every 

 reversal of the current; if a second coil is 

 placed near the first the changing magnetic 

 field of the first coil induces an electromotive 

 force in the second coil. 



If a magnetic compass is placed in an alter- 

 nating current coil, the needle is jerked back 

 and forth as the magnetic force changes di- 

 rection, but the needle cannot turn as rapidly 

 as the magnetic field changes. The result is 

 that the needle is demagnetized. The effect 

 is similar to that of hammering a magnet (see, 

 again, MAGNET AND MAGNETISM). A magnet- 

 ized watch can be demagnetized by placing 

 it in an alternating current coil. 



Another example of the magnetic action of 

 an alternating current is found in the tele- 



ELECTROMOTIVE FORCE 



phone receiver, the core of wlu'ch is a perma- 

 nent steel magnet. The effect of the feeble 

 alternating current which flows through the 

 coil is not to reverse the magnetic field, as 



AN ELECTROMAGNET 

 With many turns of insulated wire. 



with a soft iron core, but alternately to 

 strengthen and weaken the magnetic field. 



Two principles sum up briefly the relation 

 between electricity and magnetism. 



1. Electricity in motion produces a magnetic 

 field. 



2. A magnetic field in motion across an elec- 

 trical conductor produces an electromotive 

 force (which see). E.E.B. 



ELECTROMETER, e lek trom'e ter, an in- 

 strument for measuring electrical potential. 

 The gold-leaf electroscope indicates roughly 

 the difference of potential between the gold- 

 leaf and the air. Electrometers are much more 

 sensitive than the gold-leaf electroscope. (See 

 ELECTROSCOPE, for illustration.) 



The principal forms of electrometer depend 

 on the attraction of two disks of brass having 

 unlike charges. A very small change in the 

 charge causes a movement of one of the disks. 

 The force which is required to bring the disk 

 back to its original position is a measure of the 

 electromotive force (which see). 



In the quadrant electrometer a thin, flat piece 

 of aluminum, called the needle, is suspended by 

 a fiber and turns in a shallow box of brass which 

 has been cut into four quadrants. The quad- 

 rants are electrically charged, two opposite 

 quadrants being positive and the other two 

 negative. The induced charges on the needle 

 cause it to turn. E.E.B. 



ELECTROMOTIVE, e lek' tro mo' live, 

 FORCE, the force which tends to cause a flow 

 of electricity. It may be compared to the 

 pressure which causes water to flow through 



