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A New Form of Self-restoring Coherer. [Mar. 18 y 



" A New Form of Self-restoring Coherer." By Sir Oliver Lodge, 

 F.R.S. Communicated verbally March 5, — Received in 

 Manuscript March 18, 1903. 



On the general subject of the detection of Hertzian waves the 

 writer took the opportunity of the discussion on Dr. Fleming's paper 

 (p. 398), to describe briefly the latest form of coherer, which Dr. Muir- 

 head and he had brought out and always now employed in connection 

 with space telegraphy, and which their assistant Mr. E. E. Robinson 

 had helped to work out. It might be described as a development of 

 the mercury form of coherer described some years ago by Lord 

 Rayleigh, and again in a modified fashion by Mr. Rollo Appleyard. 

 In Lord Rayleigh's form this consisted of a pool of mercury cut across 

 with a paraffined knife, and the two half pools connected to a battery 

 and key. As soon as the key was depressed so as to throw a few 

 volts on to the intervening film of oil, the electrostatic pressure seemed 

 to squeeze the oil out, and the pools of mercury became one. 



Needle points clipping in oil and mercury were tried as practical 

 coherers, the points being pulled out electromagnetically every time a 

 signal arrived. Rotating forms of contact for automatic decoherence 

 were also tried in various forms, and ultimately the method took the 

 form of a rotating sharp-edged steel wheel, about half an inch in dia- 

 meter, constantly touching a pool or column of mercury on which was 

 a thin layer of oil. No effective contact occurs between the wheel and 

 the mercury, notwithstanding the immersion, because of the film of oil ; 

 but the slightest difference of potential applied to the two, even less 

 than one volt, is sufficient to break the film down and complete a 

 circuit, which, however, the rotation of the wheel instantaneously 

 breaks again. The spark is so sudden that for its purposes the wheel 

 is for the instant virtually stationary, and yet the decohesion is so 

 rapid that signals can be received in very rapid succession. The 

 definiteness of the surfaces and of the intervening layer make the 

 instrument remarkably trustworthy, and the thinness of the insulating 

 film makes it very sensitive. In fact a single cell of a battery cannot 

 be employed as a detector, because it is of too high a voltage for the 

 film to stand. A fraction of a volt is employed by a potentiometer 

 device — usually something like one-tenth of a volt — and it is adjusted 

 to suit circumstances. The battery acts through the coherer direct on 

 a low resistance recorder, and the record on the strip shows every 

 character of the arriving pulses, and exhibits any defect in the signal- 

 ling. Provided that every joint and contact, except the one intended 

 to be filmed, is thoroughly good, the coherer in this form is so definite 

 and satisfactory that it becomes safe to say that the only outstanding 

 defects are those which occur at the sending end. The signals are 



