HERTZIAN ^yAYE WIRELESS TELEGRAPHY. 207 



Another method which has been adopted by M. d'Arsonval is to 

 cause the spark to pass between two balls placed at the extremities of 

 metal rods, which are in rapid rotation like the spokes of a wheel. 

 In this case, the draught of air produced by the passage of the spark 

 balls blows out the arc and performs the same function as the blast 

 of air in Professor Elihu Thomson's method. When these adjust- 

 ments are properly made, it is possible, by means of a condenser and 

 an alternating current transformer supplied with current from an 

 alternator, to create a rapidly intermittent oscillatory discharge, the 

 sparks of which succeed one another so quickly that it appears almost 

 continuous. When using a large transformer and condenser, the 

 noise and brilliancy of these sparks are almost unbearable, and the eyes 

 may be injured by looking at this spark for more than a moment. 

 In the construction of transformers intended to be used in this man- 

 ner, very special precautions have to be taken in the insulation of 

 the primary and secondary circuits, and the insulation of these from 

 the core. 



It may be remarked in passing that experimenting with large high 

 tension transformers coupled to condensers of large capacity is ex- 

 ceedingly dangerous work, and the greatest precautions are necessary 

 to avoid accident. In the light, however, of sufficient experience there 

 is no difficulty in employing high tension transformers in the above 

 described manner, and in obtaining electromotive forces of upwards 

 of a hundred thousand volts supplied through transformers capable of 

 yielding any required amount of current. 



On occasions where continuous current alone is available, a motor 

 generator has to be employed converting the continuous current into 

 an alternating current. This is best achieved by the employment of a 

 small alternator directly coupled to a continuous current motor ; or by 

 providing the shaft of a continuous current motor with two rings con- 

 nected to two opposite portions of its armature, so that when continuous 

 current is supplied to the brushes pressing against the commutator, 

 an alternating current can be drawn off from two other brushes touch- 

 ing the above mentioned insulated rings. 



The next element of importance in the transmitting arrangement 

 is the spark gap. In the case of those transmitters employing an ordi- 

 nary induction coil, the secondary spark, or the discharge of any con- 

 denser connected to the secondary terminals can be taken between the 

 brass balls about half an inch or one inch in diameter, with which the 

 terminals of the secondary coil are usually furnished; and it is gen- 

 erally the custom to allow this spark discharge to take place in air at 

 ordinary pressure. In the very early days of his work Mr. Marconi 

 adopted the discharger devised by Professor Ehigi, in which the spark 

 takes place between two brass balls placed in vaseline or other hio-hlv ■''n"*'' *-}'^ '^ 



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