1907.] on Recent Contributions to Electric Wave Telegraphy. 687 



antenna, and then on setting the machine in operation high-frequency 

 undamped currents would be created in the antenna, and correspond- 

 ing waves radiated. To secure the best results, it is necessary, how- 

 ever, to syntonise the free-time period of the antenna circuit and the 

 natural frequency of the alternator. The chief difficulty, however, is 

 to construct a machine which shall give alternating currents of 

 sufficiently high frequency and voltage with sufficient power and cur- 

 rent capacity. Sixteen or seventeen years ago Professor Elihu Thomson 

 and M. Tesla built dynamos giving an alternating current of 10 

 amperes at a frequency of 10,000 to 15,000, and an output of about 

 1000 watts. Mr. Duddell exhibited to the Physical Society, in April 

 1005, an alternator capable of a frequency of 120,000, but its power 

 output was not more than ' 2 watt. T have on the table a small 

 alternator made by Mr. S. G-. Brown, giving an alternating current 

 having a frequency of 12,000, an E.M.F. of 20 volts, and a power of 

 about 50 watts. Professor Fessenden has recently given a description 

 of an alternator made for him having a frequency of 60,000, with 

 an output of 250 watts, running at a speed of 10,000 R.P.M., and 

 giving an E.M.F. of 60 volts. Since steam turbines of the Leval 

 type are now made to run at 500 revolutions a second, it is not 

 difficult to construct an inductor alternator having a frequency of 

 50,000 to 100,000. Such a type of alternator has, however, always a 

 large fall in terminal potential difference if called upon to give out 

 current. For this reason, a type of machine without iron in the 

 armature is to be preferred, but then it becomes more difficult to 

 balance the moving parts for very high speeds. In spite of some 

 attempts, the difficulties of making and driving a high-frequency 

 and high-potential alternator of any considerable output, say 10 kilo- 

 watt size, have not yet been overcome. Even if we could secure a 

 frequency of 50,000, this corresponds to a wave of 4 miles in length, 

 and special antenna arrangements are necessary to radiate and receive 

 such Avaves. Hence the alternator method of electric wave production 

 will certainly not supersede the spark method, although in some cases 

 it may be practicable and useful. 



In the next place we have the electric arc method to which so 

 much attention has lately been directed, employing a continuous cun'ent 

 arc with a condenser and inductance placed in series across the ter- 

 minals of the arc. As in many other cases the seeds of this invention 

 were sown in the form of discoveries by several workers. In July 1S92, 

 Professor Elihu Thomson filed a United States Patent No. 500,630, 

 in which he proposed a method for creating high-frequency alternating 

 currents by connecting a condenser and inductance to a pair of spark 

 balls and this spark gap was also connected through two other in- 

 ductances with a source of continuous current supply such as a storage 

 battery or dynamo (see Fig. 8). An air blast or magnetic field was 

 employed to continually extinguish the continuous current arc formed. 

 The operation of the arrangement was thought to be as follows. When 



