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MEASURING THE OF PARTICLES 57

tive electricity would come abruptly through the slit, and the

amount of the charge would be a measure of the number of
particles passing through the slit If instead of moving the
parabolic slit we keep the slit fixed and gradually increase the
magnetic field used to deflect the particles, we shall in this way
drive one parabola after another on to the slit, beginning with
the parabola due to the hydrogen atom and ending with that
due to the mercury one, and the charges passing the slits will
be proportional to the number of particles.

FIG. 34.

The apparatus used to carry this idea into practice is re-
presented in Fig. 34. After passing through the electric and
magnetic fields the particles, instead of falling on a photo-
graphic plate, fall on the end of a closed cylindrical metal box
B. In the end of this box nearest the cathode a parabolic
slit about i mm. In width is cut, the vertex of the parabola
being the point where the undeflected rays would strike the
box, and the tangent at the vertex the line along which the
particles would be deflected by the magnetic force alone.
This slit Is the only entry into the box. Inside the box and



	MEASURING THE OF PARTICLES 57 tive electricity would come abruptly through the slit, and the amount of the charge would be a measure of the number of particles passing through the slit If instead of moving the parabolic slit we keep the slit fixed and gradually increase the magnetic field used to deflect the particles, we shall in this way drive one parabola after another on to the slit, beginning with the parabola due to the hydrogen atom and ending with that due to the mercury one, and the charges passing the slits will be proportional to the number of particles.



	FIG. 34. The apparatus used to carry this idea into practice is re- presented in Fig. 34. After passing through the electric and magnetic fields the particles, instead of falling on a photo- graphic plate, fall on the end of a closed cylindrical metal box B. In the end of this box nearest the cathode a parabolic slit about i mm. In width is cut, the vertex of the parabola being the point where the undeflected rays would strike the box, and the tangent at the vertex the line along which the particles would be deflected by the magnetic force alone. This slit Is the only entry into the box. Inside the box and