ELECTRICAL APPARATUS. 173 



bismuth an equal quantity of heat is evolved. When a current 

 passes between two platinum plates immersed in water decom- 

 position takes place, and oxygen is evolved at the plate by which 

 the current enters the water, and hydrogen at the plate by whicn 

 it leaves ; inversion of the current inverts the chemical action. 

 Again, a current encircling a piece of soft iron renders it 

 magnetic, and if the current is inverted the magnetisation is 

 reversed. Lastly, if motion is produced by the mutual action of 

 a current and a magnet, or of one current on another, the inver- 

 sion of the current causes inversion of the motion. The only 

 two cases in which the effect of a current is independent of its 

 direction are the development of heat in a homogeneous con- 

 ductor, and the force exerted by one part of a current upon 

 another part of the same current. The distinction between the 

 former (reversible) class of effects, and the latter (non-reversible) 

 is connected with the fact that the work expended in a given 

 time in producing any of the former class is simply propor- 

 tional to the strength of the current, whereas in the case 

 of the latter class of effects it is proportional to the square 

 of the current-strength. A further distinction is that if any 

 reversible effect which a current would produce by traversing a 

 conducting circuit in a given direction, is caused by external 

 agency, a current is generated in the circuit. Thus, by supplying 

 heat at certain points of a circuit composed of alternate pieces of 

 two different metals, and withdrawing heat at certain other points, 

 a (thermo-electric) current is produced in the same direction as that 

 which would have caused heat to be absorbed at the first set of 

 points, and to be evolved at the second set. Again, if the chemical 

 action, which a current in a given direction would produce in a 

 compound liquid, is produced Ly other means, the result is a cur- 

 rent in the same direction provided the liquid makes part of a 

 conducting circuit. Similarly, work done in causing changes of 

 magnetisation in the neighbourhood of a conducting circuit, or 

 changes in the position of magnets or currents relatively to the 



