1894.] on the Work of Hertz. 343 



Also I exhibit a small complete detector made by my assistant, 

 Mr. Davies, which is quite portable and easily set up. The essen- 

 tials (battery, galvanometer and coherer) are all in a copper 

 cylinder three inches by two. A bit of wire a few inches long, 

 pegged into it, helps it to collect waves. It is just conceivable that 

 at some distant date, say by dint of inserting gold wires or powder in 

 the retina, we may be enabled to see waves which at present we are 

 blind to. 



Observe how simple the production and detection of Hertz waves 

 are now. An electrophorus or a frictional machine serves to excite 

 them ; a voltaic cell, a rough galvanometer, and a bad contact serve 

 to detect them. Indeed, they might have been observed at the 

 beginning of the century, before galvanometers were known. A 

 frog's leg or an iodide of starch paper would do almost as well. 



A bad contact was at one time regarded as a simple nuisance, 

 because of the singularly uncertain and capricious character of the 

 current transmitted by it. Hughes observed its sensitiveness to 

 sound waves, and it became the microphone. Now it turns out to be 

 sensitive to electric waves, if it be made of any oxidisable metal (not 

 of carbon),* and we have an instrument which might be called a 

 micro-something, but which, as it appears to act by cohesion, I call 

 at present a coherer. Perhaps some of the capriciousness of an 

 anathematised bad contact was sometimes due to the fact that it was 

 responding to stray electric radiation. 



The breaking down of cohesion by mechanical tremor is an ancient 

 process, observed on a large scale by engineers in railway axles and 

 girders ; indeed, the cutting of small girders by persistent blows of 

 hammer and chisel reminded me the other day of the tapping back of 

 our cohering surfaces after they have been exposed to the welding 

 effect of an electric jerk. 



If a coherer is shut up in a complete metallic enclosure, waves 

 cannot get at it, but if wires are led from it to an outside ordinary 

 galvanometer, it remains nearly as sensitive as it was before (nearly, 

 not quite), for the circuit picks up the waves and they run along the 

 insulated wires into the closed box. To screen it effectively, it is 

 necessary to enclose battery and galvanometer and every bit of wire 

 connection ; the only thing that may be left outside is the needle of 

 the galvanometer. Accordingly, here we have a compact arrange- 

 ment of battery and coil and coherer, all shut up in a copper box 

 (see left-hand side of Fig. 21). The coil is fixed against the side of 

 the box at such height that it can act conveniently on an outside 



* Fitzgerald tells ine that lie has succeeded with carbon also. My experience 

 is that the less oxidisable the metal, the more sensitive and also the more 

 troublesome is the detector. Mr. Robinson has now made me a hydrogen vacuum 

 tube of brass filings, which beats the coherer for sensitiveness. July, 1894. 



I wish to express my obligation to Mr. Edward E. Robinson for his extremely 

 competent aid with all these experiments. 



