1891] on the Work of Hertz. 337 



astonishingly sensitive detector of Hertz waves. It differs from an 

 actual air-gap in that the insulating film is not really insulating ; 

 the film breaks down not only much more easily, but also in a less 

 discontinuous and more permanent manner, than an air-gap. A tube 

 of filings, being a series of bad contacts, clearly works on the same 

 plan ; and though a tube of filings is by no means so sensitive, yet 

 it is in many respects easier to work with, and, except for very 



Fio. 17. 



Coherer, consisting of a spiral of thin iron wire mounted on an 

 adjustable spindle and an aluminium plate. When the lever is 

 moved clockwise, the tip of the iron wire presses gently against the 

 aluminium plate. 



feeble stimuli, is more metrical. If the filings used are coarse, say 

 turnings or borings, the tube approximates to a single coherer; if 

 they are fine, it has a larger range of sensibility. In every case 

 what these receivers feel are sudden jerks of current ; smooth sinuous 

 vibrations are ineffective. They seem to me to respond best to waves 

 a few inches long, but doubtless that is determined chiefly by the 



Fig. 18. 



Iron Borings Tube. One-third natural size. 



dimensions of some conductor with which they happen to be associated. 

 Figs. 17 and 18. 



I picture to myself the action as follows : Suppose two fairly 

 clean pieces of metal in light contact — say two pieces of iron —con- 

 nected to a single voltaic cell ; a film of what may be called oxide 

 intervenes between the surfaces, so that only an insignificant current 

 is allowed to pass, because a volt or two is insufficient to break down 



Vol. XIV. (No. 88.) 2 a 



