ii$ Mr. G. V. MacLean on the ' 



M. Le Rover and Paul van Berchem *, at Geneva in April 

 of 1894, used a Branly tube, containing iron filings, kept 

 between two magnetized needles. Their results showed that 

 the tube they employed had not a wave-length peculiar to 

 itself, and that it acted as an analyser and not as a resonator, 

 but that it would serve to measure the electric wave-lengths 

 in air. 



The coherer here employed is one reduced to its elements. 

 It might, in fact, be termed an electro-bolometer, and in 

 general is not more difficult to use than the heat-bolometer. 



Two globules of platinum, 1 mm. in diameter, are attached 

 to the ends of two platinum wires 0*12 mm. in diameter and 

 1'7 cm. in length. These latter form spirals, each of two 

 convolutions, about two iron terminals 1*5 mm. in diameter 

 and 4 cm. in length. These terminals run through the centre 

 of the two brass caps of a glass tube 85 cm. long and 1*5 cm. 

 in diameter. To one of the terminals was connected a micro- 

 meter screw, so that the platinum globules could be adjusted 

 to any distance from each other. This constituted the 

 coherer proper. The glass tube was not exhausted ; it served 

 merely to protect the globules from rust, dampness, or external 

 interferences. The coherer is placed in circuit with a large 

 Volla-Pavia battery of peculiar construction, giving a steady 

 current, a resistance-box in which a resistance of over 

 100 ohms was always kept, and a direct-reading milliampere- 

 meter. The current was shunted by a suitable resistance 

 before passing through the coherer. 



The capacity and self-induction of the coherer were respec- 

 tively furnished by two sheets of tinfoil 14*5 cm. by 14*1 cm., 

 shellacked to the sides of a glass plate 30"4 cm. long, 30'4 cm. 

 wide, and 5 cm. thick, and two copper wires 9"62 cm. long and 

 •3 mm. in diameter, parallel to each other and distant 5 cm. 

 These two wires had their ends soldered to two strips of brush 

 copper, which latter make close contact with the tinfoil by 

 means of hard wax; the other ends passed into two small 

 binding-posts on the caps of the coherer. The current through 

 the coherer was governed by a key. This constituted the 

 coherer or receiving circuit. All the wires used throughout 

 the whole experiment were well insulated, twisted, and kept 

 as far as possible out of the direct course of the electric waves, 

 so that any influence the wires might exert upon the results 

 was reduced to a minimum. 



The different parts of the coherer circuit can be seen in the 

 photograph (Plate I.). 



* Bibliotheqtie Univ.; Anhives de Geneve, tome xsxi. 1894, p. 558. 



