248 



NATURE 



[July 16, 1903 



of the most efficient. Most noteworthy feature of all 

 is the remarkably delicate coherer which has been 

 finally evolved from numerous experiments, a coherer 

 which not only promises to be accurate and trust- 

 worthy in practical work, but also possesses 

 several advantages from an experimental point of view, 

 a characteristic of no small importance in a piece of 

 apparatus which has to be employed in an art in which 

 there is so much to be learnt. 



In g-eneral outline the Lodge-Muirhead system does 

 not differ materially from other wireless telegraph 

 systems, a fact which is not remarkable when it is re- 

 called how much other systems owe to the pioneering 

 work which Sir Oliver Lodge has carried on ever since 

 the earliest days of Hertzian waves. In fact, if we 

 retrace the development of Hertzian telegraphy from 

 Maxwell's theory of light, the name of Sir Oliver 

 Lodge is singularly prominent, and must be associated 

 with all the more important advances. The connec- 

 tion begins in 1888, when he read a paper on the velocity 

 of electromagnetic waves along wires at the meeting 

 of the British Association, at which Prof. Fitzgerald 

 directed attention to the work that Hertz had accom- 

 plished; a little later he discovered, in its simplest 



between Elmers End and Downe there is no earth 

 connection. The precise utility of an earth connection 

 has been often in dispute, most people maintaining that 

 it merely serves to introduce the earth as the second 

 plate in a large condenser, the first plate being repre- 

 sented by the aerial wire and any capacity connected 

 to it. In the system under consideration, a second 

 capacity is provided which lies upon but is insulated 

 from the earth ; in the Elmers End station the capacity 

 was beneath the floor of the instrument shed, and was 

 connected to one terminal of the spark gap (or trans- 

 former), the other terminal being connected to the 

 aerial, which has an open wire cage serving as a 

 suitable capacity at its upper end. We need not enter 

 here into the various ways in which the circuits can 

 be connected up ; the relative positions of coherer, 

 spark gap, capacity and self-induction, the employ- 

 ment or not of the transformer, &c., offer a number 

 of solutions to the problem of designing a complete 

 station each of which has its special merits for par- 

 ticular purposes. In principle, all result in the same 

 thing — a very large Hertz radiator transmitting into 

 space a succession of untuned or carefully tuned electro- 

 magnetic waves. The two questions of primal im- 



FiG. I. — Complete Lodge-Muirhead Apparatus. 

 From left to right as follows : — Battery, receiver, spark gap, induction coil, signalling key, buzzer (at the back), automatic transmitter, and perforator. 



form, coherer action, and it is interesting to note that 

 after long trial of the filings coherers derived from 

 the discoveries of Branly, there seems to be a tendency 

 on all sides to return to simpler designs much more 

 closely resembling Lodge's original single contact 

 coherer. To Lodge also belongs the credit of having 

 been the first to insist upon the importance of tuning, 

 and of having pointed out how this might be possibly 

 attained by the proper use of self-induction and capa- 

 city. Moreover, it was, we believe, he who suggested 

 using a transformer in the aerial circuit at both trans- 

 mitting and sending stations instead of connecting the 

 spark gap or coherer direct to the aerial ; this device is 

 now in general use for tuned systems. It will readily 

 be realised, therefore, that a system which has been 

 designed by Sir Oliver Lodge is likely to be one of the 

 most promising of wireless telegraph systems, and that 

 this is all the more likely to be the case in the present 

 instance, as Sir Oliver has had the cooperation of Dr. 

 Muirhead. 



We do not propose to give a general description of 

 the system, for, as we have said, other systems are 

 similar in general outline, and with these most people 

 are by now famiUar. In the installation working 



portance are how to produce those waves, and how to 

 detect them at the receiving end. 



The production of the Hertzian waves presents several 

 difficulties. Even for moderate ranges of transmission 

 fairly powerful sparks have to be used; these are ob- 

 tained from a special induction coil and spark gap 

 (Fig. i). Here again one notices in the simple spark 

 gap between two rods a return to less complicated 

 apparatus ; in the early days of wireless telegraphy a 

 spark gap between polished balls in oil or vaseline 

 used to be regarded almost as essential. In using 

 this apparatus for syntonic work a very great deal 

 depends upon the spark. It is necessary, in the 

 first place, to obtain a regular succession of 

 sparks for every depression of the signalling key. 

 The ordinary forms of make-and-break used with in 

 duction coils have not been found satisfactory, and a 

 special form of interrupter or " buzzer," as it is called, 

 has been designed. This is seen at the back on the 

 right of Fig. i. It consists of an ordinary mercury 

 break operated by two cross-connected telegraphic 

 sounders. The first of these sounders works in the 

 same manner as an ordinary electric bell, the- arm 

 vibrating to and fro when the signalling key is de- 



NO. 



1759, VOL. 68] 



