800 The Outline of Science 



7, or with Nos. 2 and 8, and so on, at any one moment. As it is 

 impossible to go into further details here, it must suffice to say that 

 the rotation of the commutator, as the divided cylinder is named, 

 causes the pairs of segments successively to come against the 

 brushes at the instant when otherwise reversal of flow would 

 occur, and prevent reversal in the stationary part of the circuit 

 by linking each brush with a coil in which current is travelling in 

 the desired direction. To revert to the coil and permanent magnet 

 illustration, the effect is equivalent to turning the coil over be- 

 tween movements, so that the same face of the coil cuts the field 

 every time. 



Electric Circuits 



A current of electricity to be maintained requires of course 

 a complete circuit, that is to say, a current only flows between 

 two poles, just as a current flows from the zinc element of an 

 ordinary primary battery through a wire back to the carbon ele- 

 ment; if the circuit be cut, the current stops. To take the case 

 of a generator with an outside circuit. It delivers current to 

 the circuit through one brush, and receives it again through the 

 other brush ; that is, assuming the circuit to be complete. If there 

 be a gap in it anywhere- if the circuit be "open" the conditions 

 necessary for electric induction are not fulfilled, and the armature 

 will revolve easily and idly, generating no current but remaining 

 in a state of excitation only. Directly the circuit is completed, 

 however, current flows through it, its amount increasing or de- 

 creasing with the speed at which the dynamo is driven. If speed 

 were allowed to increase indefinitely the friction might ultimately 

 cause the conductor to heat up to melting-point. An ordinary 

 "circuit" is normally in a "broken" or incomplete condition. The 

 fixed conductors which form part of it are near each other, so that 

 connection may be made between them where desired. On an 

 electrified tramway one conductor is represented by an overhead 

 insulated wire, and the other by the rails which act as "earth" and 



