December 13, 1907] 



SCIENCE 



819 



the spark-gap of passing almost instantane- 

 ously from a condition of being an insula- 

 tor for electricity to being an extremely 

 good conductor for electricity is of the 

 utmost value in the spark method of wire- 

 less telegraphy. The more perfectly the 

 spark-gap insulates before the discharge 

 takes place, and the more perfectly it con- 

 ducts after the discharge has taken place, 

 the better it is for our purpose. 



If I take two electrodes sufficiently far 

 apart in air and gradually raise the elec- 

 trical pressure between them, the first indi- 

 cation that anything is going to happen is 

 the formation of fine violet aigrette on the 

 more pointed or rougher parts of the elec- 

 trodes. This is known as the brush dis- 

 charge. By gradually raising the pressure, 

 this brush discharge extends further out 

 into the air, until finally the air between 

 the two electrodes becomes so strained that 

 it breaks down and the real spark passes. 



The long thin spark that occurs in this 

 case is not very suitable for wireless teleg- 

 raphy, as its resistance is too high. Ordi- 

 nary lightning flashes are good examples of 

 long sparks on a very large scale. If in- 

 stead of working with the electrodes far 

 apart they are placed nearer together, and 

 if the electrical pressure is supplied from a 

 very powerful source, then directly the 

 spark passes it forms a thick discharge hav- 

 ing the appearance of a flame in which the 

 nitrogen of the air is actually being burnt ; 

 a process which, it is hoped, in the future 

 may have immense importance in the sup- 

 ply of artificial nitrates for agriculture. 

 This flame-like discharge has a low elec- 

 trical resistance, but has the effect that it 

 so heats or modifies the air that it is diffi- 

 cult to get the air to insulate again, after 

 one discharge, ready for the next. 



If a large quantity of electricity is dis- 

 charged through the spark-gap, and if the 

 spark lasts a very short time compared with 

 the interval between successive sparks, then 



a highly conducting spark can be obtained, 

 as well as a good insulation between the 

 sparking terminals when no discharge is 

 passing. 



In order to help to bring the gap back 

 to its insulating condition after each dis- 

 charge, many devices are employed, such 

 as subdividing the spark into several short- 

 er sparks, cooling the electrodes, blowing 

 air across the spark-gap, etc. When the 

 condenser, or antenna, discharges through 

 the spark-gap, oscillations are set up which 

 radiate Hertzian waves. 



In practise in wireless telegraphy it is 

 difficult to obtain a large number of oscilla- 

 tions during each discharge as correspond- 

 ing with each oscillation ; the antenna radi- 

 ates energy. A large number of oscilla- 

 tions means, if we keep amplitude of each 

 the same, that we are radiating a large 

 quantity of energy. Besides this radiated 

 energy, which is useful for transmitting 

 messages, there is also energy wasted in 

 heat in the spark-gap, in the conductors, 

 in the glass or other insulation of the con- 

 densers. It is this useless part which we 

 require to make as small as possible. 



I have lately had an opportunity to de- 

 termine how many oscillations actually take 

 place in a certain wireless transmission. 

 The experiment was made by photograph- 

 ing the spark as seen in a mirror rotated 

 at a very high speed, and it was found that 

 each spark consisted of nine or ten com- 

 plete oscillations. 



If all the oscillations had been of equal 

 strength or amplitude, and if the receiving 

 circuit had been similar to my pendulum 

 in my mechanical model, then there would 

 be very little to be gained by increasing the 

 number of oscillations. As the oscillations 

 die away in the spark method, two or three 

 times this number would probably be re- 

 quired for the best effect. As a matter of 

 experiment very good tuning was obtained 



