You may have observed that in the absence of any deflecting fields the position of the spot on the screen changes when the accelerating voltage is changed. One reason for this effect is the earth's magnetic field. Can you find an orientation of the tube for which there is no deflection? In this position, what is the relation between the axis of the tube and the direction of the earth's magnetic field? Use the large magnetic compass in the lab as a guide.
Now try to find a direction for which the deflection is maximum.
Using Eq. 2, compute the value for B. Note that in this case
l is the total distance from
to the screen, and L=0. Compare your
determination of the magnitude and direction of the earth's field
with the handbook values.