THE INDUCTION COIL 5 



can be seen in the figure. The number of turns of wire in this coil is 

 5,000 or more. The turns of wire in each coil are carefully insulated 

 from each other. 



The action of the coil depends upon the fact that if the strength 

 of a current running along a wire be altered, an induced current 

 is set up in a second wire placed near to it. 



The E.M.F. of the induced current depends upon several factors : 



1. It is directly proportional to the intensity of the current change 

 in the first wire. 



2. It is directly proportional to the rate of change of the inducing 

 current. 



3. It is inversely proportional to the distance between the two 

 wires. 



4. It varies with the angle between the two wires, the maximum 

 effect being produced when the wires are parallel to each other, 

 and no effect when they are at right angles to each other. 



5. The strength of the induced current may be increased by con- 

 centrating the force of the magnetic field ; as, for instance, by placing 

 a coil of soft iron wires in the interior of the primary coil. 



Some Qr all of these various factors are utilised in the production 

 of an induced current for physiological purposes ; but as the induced 

 current produced by the induction of one wire upon one other is 

 very small, the induction coil forms a very convenient means by 

 which these weak induced shocks may be multiplied and added to 

 one another. By taking a large number of turns of wire in each 

 coil the effect is greatly increased, because each turn of the primary 

 coil induces a current in each of the turns of the secondary, and all 

 these small effects are added together to produce a single greatly 

 increased effect. We have seen that an induced current is only 

 produced in the secondary coil during a change in the strength of 

 the current in the primary, so that if that change be effected 

 instantaneously, as in breaking the current, the induced current is 

 also instantaneous. The direction of the induced current is such as 

 to tend to oppose the new change, so that if a current be suddenly 

 sent into the primary coil, round which it runs in the direction of the 

 hands of a watch, the induced current in the secondary coil passes along 

 its turns in the reverse direction, i.e. against the direction of the hands 

 of a watch. Conversely, on suddenly breaking the primary current, 

 the induced current is in the same direction as that in the primary. 



In a consideration of the action of the induction coil, there is a 

 further point of some considerable importance, for just as the wires 

 of the primary can react upon the wires of the secondary coil, so can 



