WIEELESS TELEPHONY FESSENDEN". 181 



The terminals are of 40 per cent platinum-iridium. In operation the 

 apparatus is arranged to charge a condenser to a definite potential 

 and discharge it. 



Plate 6 shows forms of apparatus for oj^erating the arc in a gas 

 under pressure. 



The apparatus of figure 1 on plate 6 is also used for the arc in 

 vacuum and the critical distance arc. 



Plate 7, figure 1, shows a multiple gap with rotating electrodes, 

 brass, amalgamated zinc, and graphite being used. 



Plate 7, figure 2, shows a multiple arc gap with electrodes of dif- 

 ferent materials, the upper terminals being water cooled. 



Plate 8, figure 1, shows a condenser dynamo. 



Plate 8, figure 2, shows a general view of one type of high-fre- 

 quency alternator. It is driven by a motor and a De Laval gear. It 

 has been operated at 96,000 cycles per second, but is generally run at 

 81,700. 



Plate 9, figure 1, shows a field disk; it is 12 inches in diameter and 

 there are 300 slots on it. 



Plate 9, figiire 2, shows the armature and field coils. There are 600 

 armature slots, each containing two turns of 13 mil wire. The field 

 current is 5 amperes. The resistance of the armature is 6 ohms; 

 it gives 160 volts and about 7 or 8 amperes. Other armatures have 

 been constructed having a resistance of 4 ohms. For some work 

 double armatures are used giving about 270 volts. The output of 

 the single-armature machines at 81,700 cycles is approximately 1 

 kilowatt. The output of the double-armature machine is approxi- 

 mately 2 kilowatts. 



Other types of high-frequency alternators are under construction. 

 One type shown in plate 10, figure 1, is designed for use on ship- 

 board. The armature disk is 6 inches in diameter and two armatures 

 are used. It is arranged to be mounted on gimbals and to be driven 

 by a steam turbine connected to the steam pipe by flexible, armored 

 steam hose. The frequency is about 100,000 and the output about 

 3 kilowatts. 



Another type, which is at present being constructed by Mr. Alex- 

 anderson, to whose efforts the success of this type of generator is 

 largely due, is designed to have an output of 10 kilowatts. Designs 

 have been made for a generator of still larger size, with a calculated 

 output of 50 kilowatts and a frequency of 50,000. This machine 

 is intended for trans- Atlantic work. 



For some of these machines, instead of driving by gear or steam 

 turbine, a special 2-cycle motor has been devised, to operate at a 

 frequency of 500 cycles per second. 



The high frequency alternator method is believed to possess a num- 

 ber of advantages over other methods, inasmuch as it is set in opera- 



