99 W. H. WARREN. 
motors is generally controlled by three methods :— 
1. Varying the field strength of the motor. 
2. Providing two windings and two commutators on a 
single armature which may be connected in series or 
multiple for high and low speeds respectively. 
3. Providing a plurality of supply circuits, each of a 
different voltage, and interlinked so that the voltages 
may be added, the motor armature being connected 
across whichever of the supply circuits will produce 
the proper speed. 
{t is usual to combine the first method with one of the 
others. Where it is necessary to drive machinery at a 
constant speed, the synchronous motor using alternating 
polyphase currents is the best. Asynchronous or induc- 
tion motors are exceptional. in their simplicity of con- 
struction, ease of operation, and general reliability, 
under unfavourable conditions of care and surroundings. 
For elevators, hoists and similar service the induction 
motor is provided with a variable resistance in the 
secondary circuit, which allows its speed to be varied 
within wide limits. These motors have a powerful start- 
ing torque. For driving ordinary machine tools requiring | 
constant speed, or for crane service when suitably modified 
in design, the squirrel-cage secondary is the most suitable. 
The wiring will be the simplest for direct current distri- 
‘bution at one voltage. The three-wire direct current, and 
the three phase, and three-wire two phase alternating 
current systems are next in order of simplicity, but the 
four-wire two phase alternating and the multivoltage 
systems are more complex and costly. 
Bridge Building.—The history of the bridge builder 
dates from the period of classic antiquity, but its develop- 
ment during the nineteenth century is due to the follow- 
ing :— 
