Further, we can now see that when we apply electricity to practical purposes, we are in fact seldom using electricity itself, but other forces (that is, other combinations of gravity and levity) which we make effective by making electricity disappear. The same is true of most of the methods of measuring electricity. As a rule, the force which sets the instrument in motion is not electricity but another force (magnetism, heat, etc.) which appears in the place of the vanishing electricity. Thus the so-called intensity of an electric current is actually the intensity with which the electricity in question disappears! Electricity serves us in our machines in the same way that food serves a living organism: it gets itself digested, and what matters is the resulting secondary product.