42 LECTURES TO SCIENCE TEACHERS. 



generated by the electric current. Now I will produce the 

 reverse effect, and show you that if I drive a current of 

 electricity through these wires I shall be able to produce 

 a mechanical effect simply by the reverse action. In this 

 case a current of electricity will be passed through these 

 coils which lie near the poles of the magnet. The con- 

 sequence of this is that the coils of wire with the electricity 

 moving through them will have a tendency to rotate past 

 the poles of the magnet, and thus magnetic energy will 

 be exerted. As soon as contact is made you see the wheel 

 will rotate in the reverse direction. 



Now this is truly what we might call a reversible engine, 

 because in the first place I exerted mechanical effort to 

 produce an electric current, and in the second place I 

 cause an electric current to produce the reverse mechanical 

 effect. But in thermo-dynamics this is not what we call 

 a reversible engine, because there are certain effects pro- 

 duced by the mechanical action which are not reversed 

 when we connect it with an electric battery. For example, 

 when I turn the machine, the two wires being in contact, 

 there is a heating the whole length of this wire, and 

 consequently when reversing this effect, when I connect 

 these wires with the battery, the wires ought to be cooled. 

 But this is not the case ; they are heated as much by the 

 battery as they were by the mechanical action, and con- 

 sequently the machine as exhibited here is not a completely 

 reversible engine. 



Another case will occur to you when you take the action 

 of a steam-engine. Suppose, in place of giving heat to the 

 steam and making it do mechanical work, that you were 

 working an engine in the reverse direction, you would 

 exert mechanical energy in compressing your steam and in 

 converting it into water, and in this way you would reverse 

 the action ; instead of gaining mechanical energy from heat 

 you would be gaining a certain amount of heat from the 

 expenditure of mechanical energy. But in this case also 

 there is waste of heat from conduction and other means 

 through the cylinder, and consequently the steam-engine 

 also is not a reversible engine in the thermo-dynamical 

 sense of the word. 



Now let me try and give you an idea of a truly rever- 

 sible engine. Supposing I had two vessels at different 



