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MANAGERS AND PROJECTIONISTS 13
shunt field F — F, thus slightly strengthening the magnet, which instantly increases its magnetic flux so that the armature wires cut more lines of force, which in turn strengthens the current flowing over the shunt field.
This process continues until the normal voltage of the machine is reached, whereupon the switch connecting with the outside circuit may be closed, thus connecting the machine with its load, and lever G adjusted, until the resistance of the field rheostat limits the shunt field current flow to the value necessary to maintain the magnetic field at the strength required to maintain the desired voltage.
Modern dynamos for the most part have more than two poles, but that fact does not in the least alter the action as before set forth. The added poles merely serve to secure the same effect with a lower armature speed and with a less massive machine.
DIRECT AND ALTERNATING CURRENT.— Direct current, also known as "continuous current" (though the term is not always correctly applied. See definition p. 25) and commonly abbreviated as "D. -C.," acts or flows continuously in one direction. It is commonly considered as flowing from positive to negative. In theory the electric impulse, commonly referred to as "current flow," is outward from the positive brush of the generator, or positive pole of the battery, on one wire of the circuit, along that wire (positive wire because it is attached to the positive brush or pole) to and through the various lamps, motors, et cetera to the negative wire (negative because it is attached to the negative brush or pole of the generator) and along that wire back to the nega.tive pole of the dynamo or battery.
Alternating current, commonly referred to by the abbreviation "A. C," is the current normally generated in the dynamo armature sent out on the circuit without commutation, so that the current on the entire system reverses its direction exactly the same as it does in the dynamo armature.
WHY A. C. IS USED.— Knowing that D. C. is best for projection, and equally good or even better for incandescent lighting, and that it may be used for power purposes, the novice very naturally inquires why it is not used exclusively.
There are several reasons why A. C. is used, three of which are as follows : First, it is not deemed practical to commutate the current from the armature of a dynamo the voltage of which exceeds 500, because of the difficulty of insulating the commutator bars.