An electron is accelerated from rest through potential difference V and then enters a region of uniform magnetic field, where it undergoes uniform circular motion. The figure gives the radius r of that motion versus V1/2. The vertical axis scale is set by Is= 3.71 mm, and the horizontal axis scale is set by V, 1/2= 35.8 V1/2. What is the magnitude of the magnetic field? r (mm) 0 VI/2 Vi/2 (V1/2 Number Units