2o8 POPULAR SCIENCE MONTHLY. 



insulating oil.* But whatever advantage may accrue from using oil as 

 the dielectric in which the spark discharge takes place, when carrying 

 out simple laboratory experiments on Hertzian waves, there is no 

 advantage in the case of wireless telegraphy. The Rhigi discharger 

 was, therefore, soon discarded. If discharges having large quan- 

 tity are passed through oil, it is rapidly decomposed or charred, 

 and ceases to retain the special insulating and self-restoring character 

 which is necessary in the medium in which an oscillating spark is 

 formed. The conditions when the discharges of large condensers are 

 passed between spark balls are entirely different from those when the 

 quantity of the spark, or to put it in more exact language, the current 

 passing, is very small. In the case of Hertzian experiments, it is 

 necessary, as shown by Hertz, to maintain a high state of polish on the 

 spark balls when they are employed for the production of short waves 

 of small energy, but when we are dealing with large quantities of 

 energy at each discharge, those methods which succeed for laboratory 

 experiments are perfectly impracticable. The conditions necessary to 

 be fulfilled by a discharger for use in Hertzian wave telegraphy are 

 that the surfaces shall maintain a constant condition and not be fused 

 or eaten away by the spark, and, next, that the medium in which the 

 discharge takes place shall not be decomposed by the passage of the 

 spark, but shall maintain the property of giving way suddenly when 

 a certain critical pressure is reached, and passing instantly from a 

 condition in which it is a very perfect insulator to one in which it 

 is a very good conductor; and, thirdly, that on the cessation of the 

 discharge, the medium shall immediately restore itself to its original 

 condition. 



When using the ordinary ten-inch induction coil, and when the 

 capacity charged by it does not exceed a small faction of a microfarad, 

 it is quite sufficient to employ brass or steel balls separated by a certain 

 distance in air, at the ordinary pressure, as the arrangement of the 

 discharger. When, however, we come to deal with the discharges of 

 very large condensers, at high electromotive forces, then it is necessary 

 to have special arrangements to prevent the destruction of the surfaces 

 between which the spark passes, or their continual alteration, and many 

 devices have been invented for this purpose. The author has devised 

 an arrangement which fulfils the above conditions very perfectly for 

 use in large power stations, but the details of this can not be made 

 public at the present time. 



* It has sometimes been stated that the spark balls must be solid metal and 

 not hollow, but this is a fallacy, and has been disproved by Mr. C. A. Chant. 

 See ' An Experimental Investigation into the Skin Effect in Electrical Oscil- 

 lators,' Phil. Mag., Vol. III., Sec. 6, p. 425, 1902. 



{To he continued.) 



