1905.] Measurement of the Length of Long Electric Waves, etc. 497 



Hence M = J(L 1 -L 2 )= 3,220 cms., 



and J( L i + L2) = 46,560 



whilst from the independent measurements of L and N as above we 

 have, 



L + N = 46,385 cms. 



Hence, the^ agreement^between the last two sums is fairly close. 

 Also the coupling M/ >/LN is found to be equal to 0'57. 



It is usual to call the " coupling " of a primary and secondary coil 

 "close" if it exceeds in value 0*5, and "loose" when it is less 

 than 0-5. 



The wave or frequency meter enables us to exhibit in the form of an 

 attractive lecture experiment the well-known fact that the closing of 

 the secondary circuit of an induction coil or transformer reduces the 

 effective inductance of the primary coil. Also since it enables us to 

 determine the frequency (n) of a high frequency current, and it enables 

 us to determine also the value of the high frequency resistance K' of 

 a round sectioned copper wire of which the diameter d and steady or 

 ordinary resistance R is known, since 



-r>, Trd ,- 

 = 80^' K > 



it becomes, therefore, a useful addition to laboratory appliances. 



In Hertzian-wave telegraphy the varying power of waves of various 

 lengths to travel over land or sea surfaces is well known, and it is, 

 therefore, a practical necessity to be able to measure the wave-lengths 

 of the wave sent out. The wave-meter enables us to conduct a kind 

 of spectroscopy on a gigantic scale when we are operating with electric 

 waves hundreds of feet in length instead of fractions of an inch. 



We can by means of it discover, for instance, that a wave 300 feet 

 in length travels well over a sea surface, but will not go across a city. 

 On the other hand, the author has been able to communicate well 

 across London by means of electric waves 1000 feet in wave- 

 length. 



[Note added, February 14, 1905. The above-described instrument 

 enables us to show that, in the case of an aerial wire or antenna, as 

 used in wireless telegraphy, inductively coupled to a condenser 

 exciting circuit, even if the two circuits, open and closed, have 

 separately the same electrical time period, yet, when coupled, there 

 are two waves radiated of different wave-lengths and frequencies, 

 differing also in period from the free separate time period of each 

 circuit. This result, predicted by theory, is confirmed by experiment. 



A name is required by which to designate the instruments here 



202 



