738 Dr. Margaret Moir on Permanent Magnetism oj 



cent, hydrogen and 35 per cent, methane, it might be ex- 

 pected to exhibit some of the features of each gas, and this 

 is found to be the case though both at the higher and lower 

 voltages hydrogen is clearly dominant. Evidence has been 

 given that ignition by continuous current has possibly a 

 direct electrical origin, that by alternating current a thermal 

 origin. By this is meant that the attack on the molecule of 

 combustible gas which breaks it up for recombination with 

 oxygen, is in the one case the projection into it of electrons 

 or of ions from the poles, in the other the shattering blow of 

 high temperature collision. 



The similarity of the curves in figs. 1 to 7 for every type 

 of ignition, shows that they have a common origin. This is, 

 that in the case of least igniting sparks of short duration the 

 product of the duration and energy of the spark is constant. 



6. These results are by the nature of the case of qualitative 

 interest, for they are not independent of the nature of the 

 poles, but they indicate the order of magnitude of the energy 

 necessary to start self-ignition. 



If it is taken that there are 3 x 10 19 molecules in a cubic 

 centimetre of the gas, energy of 0*1 joule is equivalent to 

 the complete burning of 8*5 x 10 16 molecules or 2*6 cubic 

 millimetres of the combustible gas in the mixture of air and 

 methane giving perfect combustion. 



LXXVII. Permanent Magnetism of Certain Chrome and 

 Tungsten Steels. By Margaret B. Moir, M.A., D.Sc. y 

 Carnegie Research Fellow in the University of Glasgow *. 



THE magnetism of permanent magnets is a subject of wide 

 interest and of considerable importance, the value of 

 many different machines depending first and foremost on the 

 constancy of permanent magnetism. 



It is a well-known fact that of the residual magnetism 

 which a specimen holds when the applied field is reduced to 

 zero, only a part is held with such a degree of firmness that 

 it can be considered permanent. In the case of some steels, 

 the application of a very small negative magnetizing force is 

 sufficient to reduce the residual magnetism to zero, while in 

 the case of others a very considerable negative field must be 

 applied. This negative field is equal and opposite to what is 

 known as the coercive force, which can therefore be con- 

 sidered as a measure of the firmness with which the residual 



* Communicated by Professor A. Gray, F.E.S. 



