736 Prof. W. M. Thornton on the Least Energy 



Taking for comparison the voltages between 100 and 200 the 

 following values are obtained at 150 : — 



Methane : 9*5 per cent, in air by volume : — 



Poles Iron. Nickel. Copper. 



Energy of break-spark.. 0*092 0*074 0*043 joules. 



Methane : with 10 per cent, of nitrogen added and made 

 up to 9*5 per cent, in air, not shown in figures: — 



Energy of spark 0*15 — 0*09 



Coal-gas : 11 per cent, in air : — 



Energy of spark 0*068 0*049 0*02 joules. 



Thus with continuous currents and at these voltages the 

 least igniting energy ranges from *02 to *15 joule, or in 

 thermal units from -0048 to *036 gram calorie. 



3. Alternating Current. — Alternating energy-voltage curves 

 for the same gases are given in figs. 3 and 4 7 and it will be 

 seen that the energies are much greater at the same range of 

 voltage. 





Iron. 



Nickel. 



Copper. 



Methane 



0*4 joule 

 0-5 



0-15 

 023 



0-5 



0-42 



Coal-gas 





The variation of least igniting energy with voltage is now 

 greater than before, and at a frequency of 100 a second the 

 minimum energy rises to as much as 2*0 joules. The reason 

 for the very definite shape of the curves in the above figures 

 is to be found in the change of duration of the least igniting 

 flashes. These curves have for continuous current ignition 

 three parts*: a rise in duration directly proportional to the 

 voltage, a fall so that the product of duration S and voltage 

 V is constant, and a later rise approximately quadratic at the 

 higher voltages. These may be represented by (1) S==&Y, 

 (2) VS a constant, (3) V^/S a constant. It has been pre- 

 viously shown that the current-voltage curves for alternating 

 current break-sparks are approximately connected by the 

 relation that VIS or WS is constant. If this is now taken 

 to be the case there should be three parts to the energy- 

 voltage curves. In the first WS = £VW is constant; in the 



* "The Ignition of Coal-gas and Methane by momentary Electric 

 Arcs," Trans. Inst. Mining Eng. vol. xliv. Part I. fig. 6. 



