All this changed directly man succeeded in producing energy electro-magnetically by the mere rotation of material masses, and in using the water-power of the earth – itself ultimately derived from the cosmic energies of the sun – for driving his dynamos. Not only is the source of energy thus tapped practically inexhaustible, but the machines produce it without consuming on their own account, apart from wear and tear, and so make possible the almost limitless accumulation of power in one place. For electricity is distinguished from all other power-supplying natural forces, living or otherwise, precisely in this, that it can be concentrated spatially with the aid of a physical carrier whose material bulk is insignificant compared with the energy supplied.