(10 volts)/(100 ohms) = .1 ampere
The resistance of the four boys might have been 60,000 ohms, and the voltage of the secondary circuit might in that case have been, say, 150.
(150 volts)/(60,000 ohms) = .0052 ampere
How does it happen that the secondary current had a pressure of 150 volts on the boys but cannot supply even the 10 volts required by the lamp?
Perhaps we can be brought to appreciate the answer to that question best by asking ourselves some others quite like it.
Why did not the man who built our mill two generations ago locate it upon the small stream that flowed near his house? The small stream was more conveniently located for him and it has quite as much fall as he got at the foot of this lake. We sometimes express the fact by saying that the "head of water" or the water pressure was quite as much in one of these cases as the other.
One boy said that the stream sometimes gives out. Another one said that it never did have water enough to run that wheel. "Undoubtedly the trouble is with the quantity," said I, "but I want to show you that we cannot maintain the pressure unless there is sufficient quantity back of it."
Fig. 123