The analogy of the wireless transmitter of intelligence may be misleading if applied to the question of power. The practicability of wireless telegraphy depends upon the marvellous susceptibility of the "coherer," which enables it to respond to an impulse almost infinitesimally small, certainly very much smaller than that despatched by the generator from the receiving station. From this it follows, as already stated, that the analogy of apparatus designed merely for the despatch of intelligence by signalling cannot safely be applied to the case of the transmission of energy.
Making all due allowances for the prospects of advance in minimising the resistance of the atmosphere, it must nevertheless be remembered that any wireless system will be called upon to compete with improved means of conveying the electric current along metallic circuits. Electrical science, moreover, is only at the commencement of its work in economising the cost of power-cables.
The invention by which one wire can be used to convey the return current of two cables very much larger in sectional area is only one instance in point. The two major cables carry currents running in opposite directions, and as these currents are both caused to return along the third and smaller wire their electro-motive forces balance one another, with the result that the return wire needs only to carry a small difference-current. The return wire, in fact, is analogous to the Banking Clearing House, which deals with balances only, and which therefore can sometimes adjust business to the value of many millions with payments of only a few thousands. Later on it may fairly be expected that duplicate and quadruplicate telegraphy will find its counterpart in systems by which different series of electrical impulses of high voltage will run along a wire, the one alternating with the other and each series filling up the gaps left between the others.
CHAPTER IV.
ARTIFICIAL POWER.
The steam-turbine is the most clearly visible of the revolutionary agencies in motors using the artificial sources of power. In the first attempts to introduce the principle the false analogy of the water-turbine gave rise to much waste of inventive energy and of money; but the more recent and more distinctly successful types of machine have been constructed with a clear understanding that the windmill is the true precursor of the steam-turbine. It is clearly perceived that, although it may be convenient and even essential to reduce the arms to pigmy dimensions and to enclose them in a tube, still the general principle of the machine must resemble that of a number of wind motors all running on the same shaft.
It has been proved, moreover, that this multiplicity of minute wheels and arms has a very distinct advantage in that it renders possible the utilisation of the expansive power of steam. The first impact is small in area but intense in force, while those arms which receive the expanded steam further on are larger in size as suited to making the best use of a weaker force distributed over a greater amount of space.
The enormous speed at which steam under heavy pressure rushes out of an orifice was not duly appreciated by the first experimenters in this direction. To obtain the best results in utilising the power from escaping steam there must be a certain definite proportion between the speed of the vapour and that of the vane or arm against which it strikes. In other words, the latter must not "smash" the jet, but must run along with it. In the case of the windmill the ratio has been stated approximately by the generalisation that the velocity of the tips of the sails is about two and a half times that of the wind. This refers to the old style of windmill as used for grinding corn.