The velocity of an electron at the anode is determined by the p.d. over the whole distance between cathode and anode and will be greater, the greater is this p.d. If the anode battery has no internal resistance, then the p.d. is the same in both cases and equals the e.m.f of the battery. In this case the velocity of electrons reaching the anode in arrangement A and arrangement B are the same. If the internal resistance of the battery is considerable, then V = E - Ir, where I is the anode current and r is the battery's resistance. Consequently the p.d. at the anode, and so also the velocity of electrons at the anode, will be greater in the case in which the anode current is less. But the electric field at the anode is less in arrangement A than in arrangement B, since that grid is at lower potential. Therefore the anode current is less in arrangement A, and so the velocity of the electrons at the anode will be greater in that arrangement than in B.
ELECTRICITY 227