MAGNETIC EFFECT OF ELECTRIC CURRENT. 



99 



The electrical units which are now almost universally employed, the ampere, the 

 ohm, the volt, the coulomb, the henry, and the farad are. however, not the original 

 c.g.s. units, but multiples or submultiples of them The original c.g s. units as a rule 

 have no names. Therefore in this text the c.g.s. units (of the so-called "electromag- 

 netic " system) which correspond to the ampere, the ohm, the volt, etc., are desig- 

 nated by the prefix ab. Thus, we have the abampere, the abohm, the abvolt, etc. 



Definition of the abohm. A wire has a 

 resistance of one abohm when one erg of 

 'heat is generated in it in one second by 

 a current of one abampere. When H in 

 equation (2), Art. 12, is expressed in ergs, 

 t in seconds, and / in abamperes, then 

 R is expressed in abohms. 



Definition of the abvolt. An electric 

 generator has an electromotive forceof one 

 abvolt when it delivers one erg per sec- 

 ond of power with a current output of one 

 abampere [see equation (6), Art. 1 8]. 



The abvolt may be defined, on the basis 

 of Ohm's Law, as an electromotive force 

 which is capable of producing a current of 

 one abampere through a circuit of which 

 the resistance is one abohm. 



Definition of the ohm. A wire has a 

 resistance of one ohm when one joule of 

 heat is generated in it in one second by a 

 current of one ampere. The ohm is equal 

 to io 9 abohms. 



Definition of the volt. An electric gen- 

 erator has an electromotive force of one 

 volt when it delivers one joule per second 

 (one watt) of power with a current output 

 of one ampere. The volt is equal to io 8 

 abvolts. 



The volt may be defined, on the basis 

 of Ohm's Law, as an electromotive force 

 which is capable of producing a current of 

 one ampere through a circuit of which the 

 resistance is one ohm. 



Side force on an electric wire which is not at right angles to a 

 magnetic field. When an electric wire is parallel to a magnetic 

 field, no force acts on the wire. If the angle between the wire 

 and the direction of the field is 0, then the field may be resolved 

 into two components H sin 9 and H cos 0, perpendicular to 

 and parallel to the wire, respectively; the latter component has 

 no action on the wire and the former component produces the 

 side force 



F= ////sin e (29) 



If the wire is not straight, or if the field is not uniform, then one must consider the 

 force action on an element of the wire, and equation (29) becomes 



A/ 



(30) 



in which A/ is a shoit portion, or element, of the wire, H is the intensity of the field 

 at the element, 6 is the angle between H and A/, / is the strength of the current 

 in the wire in abamperes, and b.F is the force pushing on A/. This force is perpen- 

 dicular both to H and to A/. 



