332 Dynamic Theory. 



Figure 144 illustrates the production of a current in a coil of wire 

 called the conductor, by induction from the action of another coil con- 

 nected with a battery and passing a current. The apparatus is so ar- 

 ranged that the battery coil can be inserted inside of the spool of the 

 conductor coil. The current can be suddenly opened or closed by dis- 

 connecting the wire from the battery or connecting it. Suppose the 

 primary coil to be near or inside the conductor coil, and the current to 

 be suddenly made by connecting the wire of the primary coil with the 

 battery, then a momentary current will pass through the conducting 

 coil, and this current will go in a direction opposite to that which cir- 

 culates through the primary. After this there will be no further current 

 through the conducting coil until the current which has been meanwhile 

 passing through the primary, or battery coil, is suddenly disconnected 

 or opened, then there is another momentary shock or current through 

 the left hand coil, and this time the current is in the same direction as 

 the one in the right hand one. So when the connection is made in the 

 right a shock occurs in the left in the contrary direction ; when it is 

 broken, shock occurs with current in same direction. The same effect 

 is produced when the right hand coil is moved towards the left hand 

 coil, or is pulled away from it. Moving towards produces a contrary 

 current in the left coil, same as closing or making the current. Moving 

 from causes the current in the same direction. In this case the current 

 continues while the motion of the right hand coil is kept up, provided 

 the two are always so near that the left coil is within the field of force 

 of the battery coil. The principle upon which the current is produced 

 in this case is that of the conversion of molar motion into electricity by 

 the change of position of the left hand coil with reference to the field of 

 iorce in the right hand one as explained above. When both coils are 

 still and the current is suddenly opened or closed, the principle is the 

 same, the effect being as if the left hand coil were suddenly brought 

 into the presence of the current and stopped there, or suddenly taken 

 out of it; the current in the left hand coil continuing only as long as the 

 process of starting or stopping the circuit in the battery coil, a very 

 small fraction of a second. When the primary coil is moved to and 

 from the secondary one, the more rapidly it is moved the stronger will 

 be the currents induced in the secondary. If the primary is inserted in 

 the secondary and the current generated in the latter by rapidly making 

 and breaking the circuit in the former , this can obviously be done much 

 more rapidly than the primary could be inserted and withdrawn, or 

 moved in any way bodily near the secondary. As making and breaking 

 the circuit is the same as moving the coil bodily, the more rapidly it is 

 made and broken, the more powerful is the generated secondary current. 

 Hence there have been many contrivances made for this purpose. One 



