some Problems of the Mass-Spectrograpli. 527 



The electric field offers no restrictions to such a develop- 

 ment, for, once having decided the value of the deflexion # 

 and the potential at our disposal to produce it, the only 

 quantity fixed is the ratio of the length of the plates to their 

 distance apart. Their absolute dimensions are only limited 

 by considerations of convenience of construction. 



The magnetic field, on the other hand, unfortunately 

 dictates an inferior practical limit to the size of the instru- 

 ment in an unequivocal manner. We have from the original 

 exact equation for the motion of the charged particle in the 

 magnetic field of intensity H, 



where Y is the potential through which the particle has 

 fallen in the discharge tube. If we express V in volts and 

 m on the ordinary chemical scale (0 = 16) we get approxi- 

 mately 



HL/0=144(mY)i 



Now H cannot very well be greater than 17,000 gauss for 

 large pole-pieces. Actually the highest value used so far in 

 t-his work is 15,000 gauss. Taking the values of the existing 

 apparatus 0=^- radian and H = 15,000, and allowing V to 

 range from 20,000 to 50,000 volts, we find that while for 

 the hydrogen atom (m=l) the length of the field required 

 ranges from 0'45 to 1*0 cm., for mercury (m = 200) it must 

 range from 6*4 to 14' 1 cm. (The actual length is 8 cm.) 



We see therefore that, while it is possible to design a 

 mass-spectrograph of precision on a small scale to inves- 

 tigate elements of the lightness of hydrogen or helium, an 

 apparatus capable of resolving the isotopes of the heavy 

 elements must of necessity be on a considerable scale. In- 

 crease in the scale of the apparatus brings the necessity for 

 extremely low pressure and other technical difficulties in its 

 train, so that it appears probable that really great increase in 

 resolving power, as in the case of X-rays, will have to come 

 ultimately from increase in the intensity of the beam of ravs, 

 enabling extremely narrow slits to be employed. 



Summary. 



Some points raised by the performance and further design 

 of the mass-spectrograph are discussed. 



The linearity "of the mass-scale and the resolving power of 



