Relations between Electrical Measurements. 451 



was used to resist the moment produced by the action of the 

 currents. 



21. Comparison of the Electromagnetic and Electrochemical 

 action of Currents. — Currents of electricity, when passed through 

 certain compound substances, decompose them ; and it is found 

 that, with any given substance, the weight of the body decom- 

 posed in a given time is proportional to the strength of the cur- 

 rent as already defined with reference to its electromagnetic 

 effect. The voltameter is an apparatus of this kind, in which 

 water is the substance decomposed. Special precautions have 

 to be taken, in carrying this method of measurement into effect, 

 to prevent variations in the resistance of the circuit, and conse- 

 quently in the strength of the current. This subject is more 

 fully treated in Part V. §§ 53, 54. 



22. Magnetic Field near a Current. — Since a current exerts a 

 force on the pole of a magnet in its neighbourhood, it may be 

 said to produce a magnetic field (§ 6), and, by exploring this 

 field with a magnet, we may draw lines of force and equipoten- 

 tial surfaces of the same nature as those already described for 

 magnetic fields caused by the presence of magnets. 



When the current is a straight line of indefinite length, like 

 a telegraph-wire, a magnetic pole in its neighbourhood is urged 

 by a force tending to turn it round the wire, so that this force 

 is at any point perpendicular to the plane passing through this 

 point and the axis of the current. 



The equipotential surfaces are therefore a series of planes 

 passing through the axis of the current, and inclined at equal 

 angles to each other. The number of these planes is 47rC, 

 where C is the strength of the current. 



The lines of magnetic force are circles, having their centres 



in the axis of the current, and their planes perpendicular to it. 



The intensity of the magnetic force at a distance, k, from the 



current is the reciprocal of the distance between two equipoten- 



2C 

 tial surfaces, which shows the forces to be -j-— 



The work done on a unit magnetic pole in going completely 

 round the current is 47rC, whatever the path which the pole 

 describes. 



23. Mechanical Action of a Magnetic Field on a closed Con- 

 ductor conveying a Current. — When there is mechanical action 

 between a conductor carrying a current and a magnet, the force 

 acting on the conductor must be equal and opposite to that 

 acting on the magnet. Every part of the conductor is therefore 

 acted on by a force perpendicular to the plane passing through 

 its own direction and the lines of magnetic force due to the 

 magnet, and equal to the product of the length of the conductor 



