1 6 ELEMENTS OF ELECTRICAL ENGINEERING. 



ing article. The force in dynes with which the field pushes side- 

 wise on one centimeter of this wire is adopted as the numerical 

 measure of the strength of the current in the wire. This f orce-per- 

 unit-length-of-wire-per-unit-field-intensity is hereafter called, sim- 

 ply, the strength of the current in the wire ; let it be represented 

 by /. The force pushing sidewise on / centimeters of wire in 

 unit field is // dynes, and if the field intensity is 3f units intensity 

 instead of one unit intensity, the force is multiplied &C times, be- 

 coming I13{ dynes. Therefore 



F=IIW (6)* 



in which F is the force in dynes pushing sidewise on / centi- 

 meters of wire which is stretched across a magnetic field of 9-C 

 units intensity, and / is the strength of current in the wire. 



The c.g.s. unit of current and the ampere. When, in equa- 

 tion (6), Fis expressed in dynes, / in centimeters, and c^in c.g.s. 

 units or gausses, then the current strength / is expressed in 

 c.g.s. units of current. That is, the c.g.s. unit of current is one 

 dyne-per-centimeter-per-unit-field-intensity, or it is a current of 

 such strength that one centimeter of a wire in which it flows is 

 pushed sidewise with a force of one dyne when the wire is 

 stretched at right angles across a magnetic field of unit intensity. 

 The so-called practical unit of current, the ampere, is one tenth 

 of a c.g.s. unit of current. f The c.g.s. unit of current is called 

 the abampere. 



* When / and ffC are not at right angles, then 



in which a is the angle between / and &C. In this case F is at right angles to both / 

 and df as before. 



f The definition of current strength as dynes-per-unit-field-per-centimeter-length- 

 of-wire, the wire being at right angles to the field, is essentially a very concise state- 

 ment of the fundamental method of measuring current. This fundamental method of 

 measuring current is a very difficult operation. It has, however, been carried out 

 with slight modifications and the c.g.s. unit current has been found to deposit 

 0.01118 gram of metallic silver per second from pure silver nitrate. On the basis of 



