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POPULAR SCIENCE MONTHLY. 



larger vessel of acid, and lead electrodes are placed in both compart- 

 ments. The current and intermittency can be regulated by more or 

 less closing the aperture between the two regions. 



When the Wehnelt break is applied to an ordinary ten-inch induc- 

 tion coil, and the inductance of the primary circuit and the electro- 

 motive force varied until the break interrupts the current regularly, 

 and with the frequency of some hundred a second, the character of 

 the secondary discharge is entirely different from its appearance with 

 the ordinary hammer break. The thin blue lightning-like sparks are 

 then replaced by a thicker mobile flaming discharge, which resembles 

 an alternating current arc, and when carefully examined or photo- 

 graphed is found to consist of a number of separate discharges super- 

 imposed upon one another in slightly different positions. 



Many theories have been adopted as to the action of the break, 

 but time will not permit us to examine these. Professor S. P. Thomp- 

 son and Dr. Marchant have suggested a theory of resonance.* One 

 difficulty in explaining the action of the break is created by the fact 

 that it will not work if the platinum wire is made a kathode. 



Although the Wehnelt break has some advantages in connection 

 with the use of the induction coil for Eontgen ray work, its utility 

 as far as regards Hertzian wave telegraphy is not by any means so 

 marked. It has already been explained that, in order to charge a 

 condenser of a given capacity at a constant voltage, the electromotive 

 force must be applied for a certain minimum time, which is deter- 

 mined by the value of the capacity and the resistance of the secondary 

 circuit of the induction coil. If the coil is a ten-inch coil and has a 

 secondary resistance of say 6,000 ohms, and if the capacity to be 

 charged has a value say of one thirtieth of a microfarad, then the 

 time-constant of the circuit is 1/5,000 of a second. Therefore, the 

 contact with the condenser must be maintained for at least 1/500 

 of a second, during the time that the secondary electromotive force 

 of the coil is at its maximum, so that the condenser may become 

 charged to a voltage which the coil is then capable of producing. 



In the induction coil, the electromotive force generated in the 

 secondary coil at the 'break' of the primary current is higher than 

 that at the 'make,' and this electromotive force, other things being 

 equal, depends upon the rate at which the magnetism of the iron core 

 dies away, and its duration is shorter in proportion as the whole time 

 occupied in the disappearance of the magnetism is less. The Wehnelt 

 break does not increase the actual secondary electromotive force, nor 

 apparently its duration, but it greatly increases the number of times 

 per second this electromotive force makes its appearance. Hence this 

 break increases the current, but not the electromotive force in the 



* See The Electrician, Vol. XLII., 1899. 



