ELECTRIC AND MAGNETIC INDUCTIONS IN THE SURROUNDING FIELD. 293 
the level surfaces due to the seats of induction at the contacts of acid and metal, 
they will probably be somewhat as drawn in fig. 4. If now the wire is not so 
arranged as to follow with properly adjusted resistances a line of induction for these 
surfaces, but pursues an irregular course, then the level surfaces will be much 
distorted, and the distribution of the induction will be greatly altered. 
We may ascribe this alteration to a distribution of electricity along the wire, the 
quantity in any small area on the surface of the wire being equal to the difference 
between the number of tubes which have entered and the number which have left 
that area since the beginning of the system. We have a familiar example of this in 
the charging of deep-sea cables. Another example is afforded by a condenser with 
terminals connected to two points in the circuit. The plates of the condenser are 
then virtually parts of the circuit. 
The effect of a junction of two wires, say of the same diameter, but of different 
specific resistances, upon the level surface will resemble that of a charge upon the 
separating surface. This can be seen in a general way from the fact that the level 
surfaces must cut the wire with the higher specific resistance at intervals shorter than 
those at which it cuts the other wire. 
If there be an insulated conducting body, say a metal sphere, near the circuit, we 
know that in the steady state there is no electric intensity, and therefore no current 
within it; consequently there is no movement of energy and no movement of 
induction through it. We can see how this condition is arrived at. As the first 
tubes of electric and magnetic induction come up to the sphere they will enter it, and 
the parts of the electric induction tubes thus entering will be broken up, causing a 
transient current in the sphere. The parts of the tubes left in the medium will end 
on the sphere giving a negative charge on the end nearer the regions of higher 
potential, and a positive charge on the end nearer the regions of lower potential. 
This will go on until such charges have accumulated that the sphere becomes itself a 
level surface. When this point is reached no more energy can enter the sphere, and 
the parts of the magnetic tubes within it cease to move. 
The charges formed on the wire or on neighbouring conductors are to be distin¬ 
guished from ordinary statical charges in this : that their existence depends on the 
existence of the current, and therefore on the motion of magnetic induction. If the 
current is stopped by a break in the circuit, so that the motion of the magnetic 
induction ceases, the electric induction ceases and the charges are all lost. We 
should expect, therefore, to find that these charges can be described in terms of the 
magnetic motions which have occurred and are occurring in the system. 
Current produced by motion of a conductor in a magnetic field. 
We may explain by general reasoning the production of a current by motion of a 
part of a circuit so as to cut the tubes of magnetic induction. We will consider the 
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