But after Faraday had shown how to construct a magneto-electric machine, the idea of applying the "compound-interest principle," and thus converting the magneto-electric machine into the "dynamo," occurred apparently simultaneously and independently to Siemens, Varley, and Wheatstone. The first dynamo constructed by Wheatstone is still in the museum of King's College, London. Wilde employed a magneto-electric machine to generate a current which was used to excite the electro-magnet of a similar but larger machine, having an electro-magnet instead of a permanent steel magnet. The electro-magnet could be made much larger and stronger than the steel magnet, and from its armature, when made to revolve by steam power, a correspondingly stronger current could be maintained. The idea which occurred to Siemens, Varley, and Wheatstone was to use the whole, or a part, of the current produced by the armature to excite its own electro-magnet, and thus to dispense with the magneto-electric machine which served as the separate exciter. When a part only of the current is thus employed, and is set apart entirely for this duty, the machine is a "shunt dynamo;" when the whole of the current traverses the field-magnet coils as well as the external circuit, it is a "series dynamo." The apparent difficulty lies in starting the current, but a mass of iron once magnetized always retains a certain amount of "residual magnetism," unless special means are taken to get rid of it, and even then the earth's magnetism would generally induce sufficient in the iron to start the action. Commencing, then, with a slight trace of residual magnetism, the revolution of the armature generates a feeble current, which passing round the magnet coils, strengthens the magnetism, whereupon a stronger current is generated, which in turn makes the magnet still stronger, and so on until the magnet becomes saturated or the limit of power of the engine is reached, and the speed begins to diminish, or a condition of affairs is reached at which an increased current in the armature injures the magnetic field as much as the corresponding increase in the field-magnet coils strengthens it, and then no further increase of current will take place without increasing the speed of rotation. In a true dynamo the whole of the energy, both of the current and of the electro-magnets, is obtained from the source of power employed in driving the machine.