1903.] Hertzian Waves, adapted for Quantitative Work. 401 



instance, to show in a very marked manner the great effect of slight 

 differences in the surface of the spark balls. If a steady series of 

 sparks from an induction coil is passed between the spark balls of a 

 Hertz linear radiator, it will produce a steady deflection on a galvano- 

 meter connected with the above-described receiver placed at a distance. 

 If the balls are then polished, the galvanometer deflection immediately 

 increases considerably. If, on the other hand, the balls are slightly 

 smeared with oil, the galvanometer deflection decreases. If the 

 radiator is approached to the receiver, or withdrawn from it, corre- 

 sponding variations in the galvanometer deflection take place. 



Such an instrument will probably be found of great use in connec- 

 tion with the design of radiators and transmitters for Hertzian wave 

 wireless telegraphy. Up to the present it has been generally difficult 

 to ascertain whether an improvement in the signalling is due to an 

 accidental increase in sensitiveness in the coherer, or to any alteration 

 or change made in the transmitter. 



Similarly, the instrument promises to be of considerable use in the 

 investigation of the transparency or opacity of various substances to 

 Hertzian waves, not merely qualitatively, but in the determination of 

 a coefficient of absorption. Preliminary experiments of this descrip- 

 tion made with the above-described instrument seem to promise for it 

 a field of practical utility, both in the laboratory and in connection 

 with Hertzian wave telegraphy. 



In the numerous experiments which finally resulted in the construc- 

 tion of the above-described form of wave detector, it was found to be 

 essential to have the iron core in the form of a number of small 

 bundles of iron wire, each wound over with its own magnetising and 

 ■demagnetising coil. No good results could be obtained when the iron 

 core was in the form of a large bundle, say half an inch in diameter, 

 and enveloped by a single magnetising and demagnetising coil. 



Another condition of success is the short-circuiting of the fine wire 

 secondary coil during the time of magnetisation of the core. The core 

 •can be indefinitely increased in size, provided the augmentation of 

 mass is obtained by multiplying small individual cores, each consisting 

 of not more than eight or ten fine iron wires, and each wound over 

 with a separate magnetising and demagnetising coil. The electro- 

 motive force in the secondary coil can in this manner be increased as 

 much as is desired, and a very sensitive wave detector produced. The 

 commutator can be driven either by an electric motor or by any other 

 source of power. 



In conclusion, I have pleasure in mentioning the intelligent assistance 

 rendered to me by Mr. A. Blok in the experiments conducted in con- 

 nection with this appliance. 



