GENERATION OF ELECTRICITY 397 



wound dynamo. If only a part of the current goes through the 

 wires on the magnets, and the other part goes directly to the supply 

 wires, this dynamo is called the parallel-wound dynamo. If the 

 two systems are combined in one dynamo, the dynamo is said to 

 be a compound one. This is the type of dynamo usually found in 

 power-houses for generating current. 



AC Currents and DC Currents. The galvanometer will show 

 that the current changes its direction in the wires when the bar 



Fig. 27QA 



magnet changes its direction; for example, the current flows in one 

 direction when the magnet is pushed down through the coil, and in 

 the other direction when the magnet is pulled out from the coil. 

 Currents which are constantly changing their directions are called 

 alternating currents (AC), when the coil passes down in front of 

 a magnet, causing the current to flow in one direction. When the 

 coil comes up on the other side of the dynamo, by a magnet, the 

 current is caused to flow in the opposite direction. Sometimes it 

 is necessary to have direct currents (DC) in place of alternating 

 currents (AC); trolley cars, for example, use direct current. The 



