1905.] Measurement of the Length of Long Electric Waves, etc. 489 



the wave-meter circuit will be considerable. There is, therefore, some 

 difficulty in finding the position of adjustment sharply. 



Another point calls for attention. It is well known, from the theory 

 of syntonised circuits, that if two circuits having capacity and inductance 

 are brought into inductive relation to each other, the resulting complex 

 circuit has two time-periods of oscillation. Even if the two circuits 

 when separate and far removed have their time-periods adjusted to 

 equality, the resulting time-period when they are brought into inductive 

 relation to each other differs from the common value. There are, in 

 fact, two frequencies in the coupled circuit, one greater and the other 

 less than the common period. These, however, tend to equality and 

 to indentity with the free independent period of each circuit separately 

 in proportion as the mutual induction between the coupled circuits is 

 reduced.* The object, therefore, held in view in designing the 

 instrument here described was to construct one which, whilst having a 

 fairly large inductance of its own, should be capable of being associated 

 with the circuit to be tested, and set in action by it, by means of a 

 mutual inductance as small as possible. 



This has been achieved by making the part of the circuit of the 

 instrument which is acted upon inductively by the circuit to be tested 

 only a small part of the circuit on which its whole inductance depends. 

 We are thus able to keep the self-inductance large and the mutual 

 inductance small, and therefore prevent any great reaction of the 

 secondary current upon the circuit which is being tested. 



A closed circuit wave-meter has been devised on these lines by the 

 author, employing a neon vacuum tube detector, which enables the 

 measurement of the oscillation-constant of the transmitter circuit to 

 be made with great accuracy and quickness, and is also useful for 

 measuring small inductances and capacities.! 



It is constructed in one form, as follows : An ebonite tube, about 

 100 cms. long, has cut upon its outer surface a screw groove having 

 1 '6 turns per centimetre and a length of 60 cms. is cut. In this groove 

 is wound a bare copper wire, SS. No. 14 S.W.G. size (see fig. 1), 

 one end of the wire is attached to a metal pin pi which forms a bearing 

 supporting the tube, and the other end of the wire is insulated. To 

 this latter end of the ebonite tube is attached a brass tube I 60 cms. 



* See A. Oberbeck, " Ueber den Verlauf der Electrischen Schwingungen bei den 

 Tesla'schen Versuchen," ' Wied. Annalen der Physik,' vol. 55, p. 627, 1895. 



t The use of a neon vacuum tube as a sensitive indicator of a high-frequency 

 electric field was mentioned first in a paper read by the author to the British 

 Association at Cambridge, September, 1904. The author is indebted to Sir 

 William Ramsay for two tubes of this gas. Sir James Dewar has shown that 

 such neon vacuum tubes can be readily prepared from atmospheric air by the 

 employment of absorbent charcoal at very low temperatures. It would be an 

 advantage if the manufacture of these neon tubes could be placed on a commercial 

 basis, in view of their utility for the purposes here described. 



