WIRELESS TELEPHONY — PESSENDEN. 173 



From 1898 to 1900 numerous experiments were made on antennae 

 of large capacity, and it was found that instead of using sheets of 

 solid metal or wire netting, single wires could be placed at a con- 

 siderable fraction of the wave-length apart and yet give practically 

 the same capacity effect as if the space between them were filled with 

 solid conductors. 



From other investigations on the variation of radiation with fre- 

 quency the result was arrived at that it should be possible to con- 

 struct an alternating-current dynamo of sufficiently high frequency 

 and output to give ample radiation for wireless telegraphic purposes." 



In 1900 a large American electrical manufacturing company kindly 

 consented to take up the construction of such a dynamo. As a pre- 

 liminary, a dynamo of 1 kilowatt output and 10,000 cycles (shown in 

 pi. 1, fig. 1) was built in 1902. By the summer of 1906 many of the 

 difficulties had been overcome, and a machine giving 50,000 cycles was 

 installed at the Bpant Rock station. Various improvements were 

 made by the writer's assistants, and in the fall of 190G the dynamo 

 was working regularly at 75,000 cycles, with an output of half a kilo- 

 watt, and was being used for telephoning to Plymouth, a distance 

 of approximately 11 miles. In the following year machines were 

 constructed having a frequency of 100,000 cycles per second and out- 

 puts of 1 and 2 kilowatts. 



The credit for the development of this machine is due to Messrs. 

 Steinmetz, Haskins, Alexanderson, Dempster, and Geisenhoner, and 

 also to the writer's assistants, Messrs. Stein and Mansbendel. 



CLOSED TUNED CIRCUITS. 



In 1898 the open tuned circuits originally used were discarded for 

 closed tuned circuits,^ and it was discovered that valuable selective 

 effects could be obtained by placing the condenser in shunt to the 

 inductance, instead of in series with it.^ 



COMBINATION OF WAVE AND GROUP TUNING. 



The fact that if selectivity is obtained solely by tuning to wave 

 frequencies, the number of stations is limited, was appreciated at an 

 early date. In 1900 " a new method was developed, the stations being 

 tuned both to the wave frequency and to an independent or group 

 frequency, so that stations might obtain selectivity by varying either 

 the wave or the group frequency and thus have at their disposal a 



« United States patent No. 70G737, May 29, 1901. 



'' United States patents Nos. 70G735 and 706730. Deeeuiljer 15, 1899. 



" United States patents Nos. 727325, June 2, 1900, and 727330, March 21, 1903. 



