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BELL SYSTEM TECHNICAL JOURNAL 



Equation (2) does not involve the mass at all. In the usual loose 

 phrasing, Hp gives the momentum of the particle provided that its charge 

 is known. The like cannot be said for the energy, which is given by 

 Hp only if both the charge and the rest-mass are known. For particles 

 of the cosmic rays it is best to disregard the ordinary expression for 

 kinetic energy {^mv"^) and adopt for good the relativistic expression 

 mc^, to wit, moc'^l^l — v^/c^. Of this the portion wqC^ is not kinetic 

 energy: it is the "rest-energy" associated with the "rest-mass" Wo, 

 inseparable from the particle so long as this exists; it amounts to about 

 half-a-million electron-volts or 0.5 Mev for the electron, to about 

 1000 Mev for the proton. The remainder may be called kinetic 

 energy. For nearly all of the electrons and most of the other cosmic- 

 ray particles, this remainder is by far the greater part. The de- 

 pendence of the kinetic energy upon Hp is exhibited, for electrons 

 and for protons, by Fig. 13 (page 213). One sees that for different 



Fig. 9 — Track exhibiting measurable and unequal curvatures on the two sides of 

 a metal plate, thus indicating changes of energy and momentum suffered in the 

 traversal. (Anderson) 



masses a given //p-value leads to dififerent energy-values, but also 

 that the error due to an incorrect estimate of rest^mass becomes 

 proportionately smaller as the iJp-value increases. Yet the possi- 

 bility of the error is always there, if the mass of the particle is not 

 certainly known; and it affects many published "energy-values" 



