In the cases here considered it is in reality the sudden presentation of the coil (twice at each rotation) before the positive and negative poles of the magnet, which induces a momentary but intense current of electricity. The rotation being exceedingly rapid, these currents succeed each other with sufficient rapidity to be appreciably continuous. A similar principle is involved in the use of what is called the inductive coil, except that in this case the sudden beginning and ceasing of a current in one coil (and not magnetic action) induces a momentary but strong current: matters are so arranged that the current induced by the starting of the inducing current, immediately causes this to cease; while the current induced by the cessation of the inducing current immediately causes this current to begin again: so that by a self-acting process we have a constant series of intense induced currents, succeeding each other with great rapidity, so as to be practically continuous, as with those produced by magneto-electric and dynamo-electric machines. All that I have said about the voltaic arc, the incandescence resulting from resistance to the current's flow, and so forth, in relation to electricity generated by galvanic batteries, applies to electricity generated by induction coils, or by magneto-electric and by dynamo-electric machines. Only it is to be noticed that in some of these machines the currents alternate in direction with each revolution of the swiftly turning coil, in others the currents are always in the same direction, and in yet others the currents may be made to alternate or not, as may be most convenient. We have now to consider how light suitable for purposes of illumination may be obtained from the electric current. Hitherto we have considered only light such as might be used for special purposes, where a bright and very intense light was required, where expense and complexity of construction might not be open to special objections, and where in general the absolute steadiness of the light was not an essential point. But those who have seen the electric light used even by the most experienced manipulators for the illustration of lectures will know that the light as so obtained, though of intense brilliancy, is altogether unsuited for purposes of ordinary illumination. If we consider a few of the methods which have been devised for overcoming the difficulties inherent in the problem of electric lighting, the reader will recognise at once the nature of these difficulties, and the probability of their being effectually overcome in the future, for though much has been done, much yet remains to be done in mastering them. Let us consider first the Jablochkoff candle, the invention of which brought about, in July 1877, the first great fall in the value of gas property.