52 POPULAR SCIENCE MONTHLY. 



HERTZIAN WAVE WIRELESS TELEGRAPHY, VII. 



By Dr. J. A. FLEMING, 



PROFESSOR OF ELECTRICAL ENGINEERING, UNIVERSITY COLLEGE, LONDON. 



A NUMBER of more or less imperfect arrangements, having the 

 -^--^ isolation of communications for their object, have been devised or 

 l-»atented, which are dependent upon the use of several aerials, eacli 

 supposed to be responsive only to a particular frequency ; and attempts 

 have been made to solve the problem of isolation by MM. Tommasi, 

 Tesla, Jegon, Tissot, Ducretet and others. 



We may then pass on to notice the attempts that have been made 

 to secure isolation by a plan which is not dependent on electrical 

 syntony. One of these, which has the appearance of developing into a 

 practical solution of the problem, is that due to Anders Bull.* In tbe 

 first arrangements proposed by this inventor, a receiver is constructed 

 which is not capable of being acted upon merely by a single wave or 

 train of waves or even a regularly spaced train of electric waves, but 

 only by a group of wave trains which are separated from one another 

 by certain unequal, predetermined intervals of time. Thus, for instance, 

 to take a simple instance, the transmitting arrangements are so de- 

 vised as to send out groups of electric waves, these wave trains follow- 

 ing one another at time intervals which may be represented by the 

 numbers 1, 3 and 5; that is to say, the interval which elapses between 

 the second and third is three times that between the first two, and the 

 interval between the fourth and fifth is five times that between the first 

 two. This is achieved by making five electric oscillatory sparks with a 

 transmitter of the ordinary kind, the intervals between which are settled 

 by the intervals between holes punched upon strips of paper, like that 

 used in a WHieatstone automatic telegraphic instrument. It will easily 

 be understood that by a device of this kind, groups of sparks can be 

 made, say five sparks rapidly succeeding each other, but not at equal 

 intervals of time. One such group constitutes the Morse dot, and 

 two or three such groups succeeding one another very quickly con- 

 stitute the Morse dash. These waves, on arriving at the receiving 

 station, are caused to actuate a punching arrangement by the inter- 

 mediation of a coherer or other kumascope, and to punch upon a uni- 

 formly moving strip of paper holes, which are at intervals of time 

 corresponding to the intervals between the sparks at the transmitting 

 station. This strip of paper then passes through another telegraphic 

 instrument, which is so constructed that it prints upon another strip 

 *See The Electrician, Vol. XLVI., p. 573, February 8, 1901. 



