Vacuum tube with screen shows cathode rays bent with a magnet.

Vacuum tube with screen shows cathode rays bent with a magnet. An evacuated tube has an anode at one end, a cathode at the other, and a fluorescent screen in between. When a high voltage (about 40 kV pulsating D.C.) is placed across the tube, a beam of electrons is emitted from the cathode, passes through a slit, then travels in a straight line to the anode. When a horseshoe magnet is lowered down over the tube, the beam of electrons is deflected. (By the 'right-hand screw rule', the direction of the deflection is VxB. So, the deflection of the beam is down, if the North pole of the magnet is coming out of the page...) The beam of electrons impinges on the fluorescent screen, making the path of the beam visible.
UCB Index: 
D+30+24
PIRA Index: 
5H30.15
Demo Diagram: 

Popularity:

Comments

Mel Pomerantz's picture

The text says that with N pointing into page deflection is down, because F = v x B. This neglects vital fact that the charges that are moving are electrons; negative charges. The conventional current, v, is from left to right. Thus q v x B is UPWARD.