S8 POPULAR SCIENCE MONTHLY. 



however, is not an important matter. Far more important is it to 

 show, as has been shown, that practically perfect isolation can be 

 achieved if it is desired. 



It must be noted, however, that, although the fact that electric cir- 

 cuits have a natural time-period of oscillation of their own is a scientific 

 principle which carries us a considerable way towards a solution of 

 what is called syntonic Hertzian wave telegraphy, it is not in itself 

 alone in every respect an entire solution of the practical problem. The 

 degree to which it is a solution depends to a considerable extent upon 

 the nature of the detecting device, or kumascope, which we are employ- 

 ing. The coherer, or Branly filings tube, has the peculiarity that its 

 passage from a nonconductive to a conductive condition follows imme- 

 diately when the difference of potential between its ends is made suffi- 

 ciently great. In other words, if the tube is acted upon by a sufficient 

 electromotive force, it is not necessary that electromotive force shoidd 

 be repeated at intervals to make this particular form of kumascope 

 responsive. Again, if we consider the nature of the oscillations which 

 are sent out from any transmitting aerial, we find that each group of 

 oscillations corresponding to a single spark consists of waves gradually 

 decreasing in amplitude. In other words, the first wave of the group 

 iii the strongest, and the decay in amplitude is often very rapid. 

 Supposing, then, we construct a simple receiver consisting of an aerial 

 having inserted in its circuit a sensitive Branly filings tube. Such a 

 receiver is almost entirely non-syntonic ; that is to say, it is affected 

 by any wave passing over it which is sufficiently powerful. We may 

 look upon it that if the first wave of the series is sufficiently powerful 

 to affect the kumascope, the conductive change takes place whether or 

 not the first wave is followed by others. Accordingly, it is perfectly 

 certain that if a transmitter is sending out trains of waves of any 

 period, a simple combination of coherer and aerial will be infiuenced, if 

 it is placed near enough to the transmitter. On the other hand, it is 

 possible to combine a kumascope of a certain type with a receiving 

 aerial and other circuits in such a manner that when the waves that 

 reach it are feeble it shall not respond at all unless those waves have 

 very nearly a time period of a certain value. 



At this stage, it may be perhaps well to explain a little in detail 

 what is meant by an easily responsive circuit, and, on the other hand, 

 by an irresponsive circuit, or, as we may call it, a stiff circuit. Sup- 

 posing that we consider an aerial consisting of a simple straight wire 

 having small capacity and small inductance, such a circuit admits of 

 being sent into electrical oscillation, not only by waves of its own natu- 

 ral time-period, but by the sudden application of any violent electro- 

 motive impulse. If, on the other hand, we bestow upon the circuit in 

 any way considerable inductance, we then obtain what may be called a 

 stiff or irresponsive circuit, which is one in which electrical oscillations 



