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 winch 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 bearings 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. 



Critical 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 



