THE MAGNETO TELEPHONE 119 



(g) Resistance The Ohm. The opposition offered by any substance to the flow 

 of the electric current through it is known as resistance. Different substances oppose 

 the flow of electricity in different degrees, copper being one that opposes a very low 

 resistance (see Conductor). Insulators oppose an immensely powerful resistance. 

 With any given conductor, increase in length causes increase in resistance; increase 

 in cross sectional area causes decrease in resistance; and, for most conductors, rise in 

 temperature causes increase in resistance. For any one conductor, resistance is, in a 

 way, analogous to the frictional resistance offered to the flow of water through a pipe. 

 Electro-motive force tends to maintain a flow of electricity against the electrical resis- 

 tance of a conductor just as a "head" of water tends to maintain a- flow of water 

 against the frictional resistance of a pipe. The unit of electrical resistance is the 

 ohm. Conductivity is the opposite of resistance. 



(/&) Current Strength The Ampere. The rate of flow of electricity is termed its 

 current strength. It is the result of E.M.F. acting through a conductor and over- 

 coming resistance, and is measured in amperes. The ampere, or unit of current 

 strength, is analogous to the " miner's inch " used in the measurement of the flow of 

 water. Obviously the three factors, E.M.F. , resistance, a id current a:-e interdepend- 

 ent. Their relation to one another is stated in Ohm's Law thus : 



The Current is equal to the Electro-motive force divided by the Resistance, or 



E 



= - 

 R 



(*') Electro-magnetic Induction. Every magnet and every current-bearing wire 

 is surrounded by a magnetic field having among other properties that of being able to 

 induce magnetism in a piece of iron or steel placed within it, and of being able to 

 produce a difference of potential between the ends of a wire moved across such a 

 magnetic field in such a way as to cut its lines of force. If the ends of this wire are 

 connected outside the magnetic field, a current will flow as long as motion is main- 

 tained. Such currents are called induced currents and the process by which they are 

 produced is called electro-magnetic induction. 



(j) Induction Coil.- An induction coil is a device consisting of two distinct coils 

 of insulated wire, one placed around the other, but not electrically connected, by means 

 of which, as a rule, currents of low potential and high amperage are changed to a high 

 potential and low amperage, or vice versa. One of the coils, usually having an iron 

 core, is used to produce the magnetic field by means of a current from a battery to 

 which it is connected. This is called the primary. The other coil, generally above the 

 primary or around, it, is called the secondary. Currents may be produced in the 

 secondary by means of any of the following methods : 



(1) By moving either the primary or the secondary while a current is flowing in 

 the primary, thereby altering the position of the coils with respect to each other. 



(2) By making or breaking the primary circuit. 



(3) By altering the current in the primary. 



(4) By reversing the direction of current in the primary. 



(5) By moving the iron core while current flows in the primary, thereby altering 

 the magnetic field. 



Any one of these operations causes a disturbance in the magnetic field as a result 

 of which currents are induced in the secondary. 



In the induction coil, as usually employed, either alternating or interrupted 

 currents in the primary induce currents in the secondary whose E.M.F. bears the same 

 relation to the E.M.F. of the primary current as the number of the turns of wire in 

 the secondary coil bears to the num'ber of turns in the primary. For example, if there 



