✔ 最佳答案
1. This just follow directly from Ohms' Law.
Let E be the emf of the power supply
r be the internal resistance
R be an external load resistance
I be the current through the load
Hence, E = I(r+R)
i.e. E = Ir + I.R
But what you could measure is I.R, which equals to the terminal voltage V of the power supply
Therefore, E = Ir + V
or E - V = Ir
The p.d. [I.r] cannot be measured directly
2. The high resistance is to limit the power output of the EHT supply. Since a EHT supply provides a high voltage, a small current (in order of mA) could provide quite a large power already.