208 



ANNUAL EEPOKT SMITHSONIAN INSTITUTION, 1915. 



If the signal is to have individual quality, so .as to be readily dis- 

 tinguishable from other noises, and so as to be separable by resonance 

 from other notes, each dot must consist of at least 10 impulses. 



Thus we arrive at the conclusion that w^hatever device is used it 

 must be capable of producing at least 100 compressional waves in a 

 single second in order to telegraph satisfactorily at the rate of 20 

 words per minute. 



If this same apparatus is to transmit speech through the water, 

 it must be still more rapid in its action and must be capable of pro- 

 ducing several thousand compressional waves per second. 



The above were 

 the three main dif- 

 ficulties in the way. 

 Of course there 

 were many others; 

 for example, the 

 apparatus must not 

 weigh too much; it 

 must not be affected 

 by water or change 

 of temperature ; it 

 must be simple in 

 construction; it 

 must be easily ap- 

 plied to the ship; 

 positive in its ac- 

 tion; must not re- 

 quire ad j ustment 

 after being once set 

 up; and must be 

 able to stand all 

 kinds of ill treatment at the hands of unskilled operators. It will 

 be unnecessary to go over the ground taken by the development, and 

 we will therefore proceed at once to describe the apparatus as finally 

 developed by Prof. R. A. Fessenden. 



The device used is termed an " oscillator," and its construction is 

 shown in cross section in the drawing, figure 1. 



In the drawing the iron of the magnetic circuit and the copper 

 tube are shaded. The magnetizing coil is crosshatched. The moving 

 part is the copper tube A. This lies in the air gap of a magnetic 

 field formed by a ring magnet B, built up in two parts, as shown in 

 longitudinal section in figure 2. 



The ring magnet is energized by the coil C, and produces an in- 

 tense magnetic flux which flows from one pole of the ring magnet 

 across the air gap containing the upper part of the copper tube, 



Fig. 1. 



