22 Recent Advances in Electricity. 



Hertz, though quite a young man, died a year or two after 

 the publication of his remarkable results, which form an era in 

 the history of electricity, but his work has been zealously taken 

 up by others, and his methods of experiment have been in some 

 respects improved. The favourite method now of obtaining 

 Hertzian waves is to discharge the secondary coil of a RuhmkorflF 

 through three metal knobs, of which the two outside ones, which 

 are connected with the ends of the coil, are small, and the 

 middle one much larger; the discharges being taken one at a 

 time by hand, instead of being allowed to run on rapidly by 

 the automatic make and break. As a receiver or analyser, to 

 reveal the presence of the waves, one of the best methods is to 

 employ a short tube of glass, filled with metal filings (called 

 "Branly's coherer"), and to make this mass of filings form part 

 of the circuit of a battery and galvanometer. When Hertzian 

 waves fall upon the filings they give an instantaneous increase 

 of conductivity, so that the galvanometer shows a stronger 

 current. An improvement on this has been devised by Prof. 

 Chunder Bose, of Calcutta, and consists in using, instead of a 

 mass of filings, a row of small spiral springs, which are held at 

 the ends by fixed supports, and touch each other at the sides 

 with a contact which can be made more or less close by an 

 adjusting screw, putting on more or less pressure. 



Hertz showed that his rays could be reflected from metal 

 plates, that they could be brought to a focus by a concave 

 reflector, and that they could be refracted through a prism of 

 suitable material. One of the best materials is pitch, which, 

 though opaque to luminous waves, is transparent to Hertzian 

 waves. Hertz also found that his waves were able to pass through 

 brick walls and through floors and ceilings. In connection 

 with such facts as these, we must remember that a piece of 

 ordinary red glass is opaque to yellow and green light, though 

 transparent to red. It is a question of the length of the waves ; 

 the yellow and green waves are not of the right length to get 

 through. Hertzian waves are many thousands of times longer 

 than light waves, and it is not surprising that media which are 

 opaque to the one are transparent to the other. 



