=Faraday’s Machine.=--After various experiments, Faraday made his “new electrical machine” as shown in fig. 126. This piece of apparatus is preserved and was shown in perfect action by Prof. S. P. Thompson in a lecture delivered April 11th, 1891, after an interval of sixty years. It consists of a horse shoe magnet and a copper disc attached to a shaft and supported so as to turn freely. The magnet is so placed that its inter-polar lines of force traverse the disc from side to side. There are two copper brushes, one bears against the shaft, and the other against the circumference of the disc. A handle serves to rotate the disc in the magnetic field. Now, if the north pole of the magnet be nearest the observer and the disc be rotated clockwise, the current _induced_ in the circuit will flow out at the brush which touches the circumference, and return through the brush at the shaft. =Faraday’s Principle.=--The principle deduced from Faraday’s experiment may be stated as follows: _When a conducting circuit is moved in a magnetic field so as to alter the number of lines of force passing through it, a current is induced therein, in a direction at right angles to the direction of the motion, and at right angles also to the direction of the lines of force, and to the right of the lines of force, as viewed from the point from which the motion originated._ Faraday’s principle may be extended as follows to cover all cases of electromagnetic induction: _When a conducting circuit is moved in a magnetic field, so as to alter the number of lines of force passing through it, or when the strength of the field is varied so as to either increase or decrease the number of lines of force passing through the circuit, a current is induced therein which lasts only during the interval of change in the number of lines of force embraced by the circuit._ =Ques. Explain just what happens when a current is induced by electromagnetic induction.= Ans. In order to induce an electromotive force by moving a conductor across a uniform magnetic field, it is necessary that the conductor, in its motion, should so cut the magnetic lines as to alter the number of lines of force that pass through the circuit of which the moving conductor forms a part. =Ques. What is the proper name for a “conductor” which moves across the magnetic field?= Ans. An _inductor_, because it is that part of the electric circuit in which induction takes place. In the case of a dynamo, an inductor may be either a copper wire or copper bar. =Ques. How may a conducting circuit be moved across a magnetic field without having a current induced therein?=