1899.] on Coherers. 73 



The electrified dust and smoke experiment, whereby a thick fog 

 in a chamber can be cleared by the discharge from a point, as 

 observed by Lodge and Clark in 1883, was also shown ; and the 

 lightning guard experiment with a couple of Leyden jars and a 

 galvanometer and two surfaces in light contact, by which the lecturer 

 observed electrical cohesion of metals in 1888, was exhibited, 

 together with a couple of resonant Leyden jars, one of them charged 

 and sparking, the other responding by closing the circuit of a local 

 battery and ringing a bell ; which bell, by its vibration, could effect 

 the tapping back. 



The discovery that this property of metals served as the best 

 detector for Hertz waves was made by Monsieur Edouard Branly, 

 Professor of Physics in the Catholic Institute of Paris ; and some of 

 Monsieur Branly's original apparatus was exhibited, especially a 

 piece of ebonite smeared over with porphyrised copper so as to form 

 a high resistance, which fluctuated in value between certain limits 

 under the influence of alternate electric sparks and tapping. A 

 Branly filings-tube connected to a speaking galvanometer was shown 

 receiving signals from a Hertz emitter at a distance, the coherer being 

 tapped back automatically in one of many alternative ways. A 

 recent experiment of Signor Tomasina, displaying the effect of 

 electric cohesion was projected on the screen : — A vertical wire about 

 nine inches long had its end immersed in filings and was slowly 

 raised. If a sphere of suitable size were sparking in the neighbour- 

 hood, between polished knobs, the electric jerks collected by the 

 vertical wire would give, as Hertz showed, minute perhaps ultra- 

 microscopic sparks to anything brought close to its end. The filings 

 subjected to this action are found to cohere, and can be pulled up 

 in a narrow string, of length depending on the steadiness of the 

 movement. 



(M. Tomasina has recently repeated this experiment under liquid, 

 and obtained chains of filings several inches long.) 



Another variety of the cohesion experiment under electrical in- 

 fluence, was shown by the lecturer in a form which suggested that 

 electrostatic attraction played a considerable part. A very fine 

 platinum wire was suspended in a glass box, with its lower end close 

 to a flat and highly polished facet of a brass knob. On looking at 

 this wire under a microscope, it and its image in the polished face 

 could both be seen, a slight distance apart ; or they could be projected 

 with a strong lens upon a screen. Under the action of electric waves, 

 the gap between the wire and its image sharply disappeared, and the 

 wire was seen clinging to the knob until tapped back. A wire of 

 this kind constitutes an extremely sensitive electroscope, but for this 

 purpose the coherence which sets in (unless silver or some such non- 

 cohering metal is employed) is inconvenient. 



Finally a layer of filings on a horizontal glass surface, with tin 

 foil electrodes, was projected on the screen, and subjected to strong 

 electric influence, under which they were seen to move so as to close 



