136 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 
millimeter) in diameter, is wound in a continuous wave up and down 
the successive slots. The peripheries of the armature frames are 
threaded to screw into the iron frame of the alternator. By means of 
a graduated scale on the alternator frame, the armatures can be 
readily adjusted for any desired air gap. 
Inductor.—The inductor or rotor has 300 teeth on each side of its 
periphery, spaced 0.125 inch (3.17 millimeters) between centers. 
The spaces between the teeth are filled with U-shaped phosphor- 
bronze wires, securely anchored, so as to withstand the centrifugal 
force of 80 pounds (36.3 kilograms) exerted by each. Since each 
tooth of the inductor gives a complete cycle, 100,000 cycles per 
second are developed at 20,000 revolutions per minute. The diame- 
ter of the disk being 1 foot (0.30 meter), the peripheral speed is 
1,047 feet (319 meters) per second, or 700 miles (1,127 kilometers) per 
hour, at which rate it would roll from the United States to Europe 
in four hours. By careful design and selection of material, a factor 
of safety of 6.7 is obtained in the disk, although the centrifugal 
force at its periphery is 68,000 times the weight of the metal there. 
Bearings.—The generator has two sets of bearings, as shown in the 
illustrations, the outer set being the main bearings which support 
the weight of the revolving parts. These bearings are self-aligning 
and are fitted with special sleeves, which are ground to coincide 
with longitudinal corrugations of the shaft, thus taking up the end 
thrust. A pump maintains a continuous stream of oil through these 
bearings, thus allowing the machine to be run continuously at full 
speed without troublesome heating. 
The middle bearimgs normally do not touch the shaft, but take up 
excessive end thrust and prevent excessive radial vibration of the 
flexible shaft. 
An auxiliary bearing or guide is placed midway between the gear 
box and the end bearing. Its function is to limit the vibration of 
that portion of the shaft. 
Oritical periods.—In starting the machine, severe vibration occurs 
at two distinct critical speeds, one at about 1,700 and the other at 
about 9,000 revolutions per minute. ‘The middle bearings prevent 
this vibration from becoming dangerous. 
Voltage.—With the normal air gap between the armatures and 
revolving disk of 0.015 inch (0.38 millimeter), the potential developed 
is 150 volts with the armatures connected in series. It is possible, 
however, to decrease the air gap to 0.004 inch (0.10 millimeter) for 
short runs, which gives a corresponding increase in voltage up to 
nearly 300 volts. It is considered inadvisable, however, to run with 
this small air gap for any considerable length of time. 
The machine is intended to be used with a condenser, the capacity 
reactance of which balances the armature induction reactance, which 
