i 103 
structure, but it gives no information as to the relative effects of big and small charges 
on the same structure. If a second rule can be established covering this side of the 
problem, the two together will obviously cover a very wide ground. 
Hopkinson’s rule can be deduced theoretically as an extension of the principle 
described in Section 9. Its validity has been proved experimentally for charges 
differing very widely in ‘magnitude. The rule is obviously of great value in enabling 
full-scale inferences to be drawn from model experiments. ‘ 
