It will be remembered that when we were studying the dynamo we produced an electric current by moving a magnet. We may now add that an electric current may be produced by simply changing the strength of a magnetic field. The coil that we have just made creates a magnetic field in the region about itself whenever a current is passing through it. The tongue at _T_ (Fig. 117) detects an extra current while the magnetic field is being produced, or while it is dying away, or it will detect any slight variations in the strength of the current which produces the magnetic field. It is customary to distinguish between these two currents. The battery current which produced the magnetic field is called the primary current and the current which is detected by the tongue is called the secondary current. The primary current in our experiments had only a few volts of pressure, from one to seven. The secondary current had many volts, as indicated by the spark. If we rub the end of the wire _c_ across the binding post under _b_ (Fig. 117) no spark occurs. The current does not in this case go through the coil, and no secondary current is produced. Whenever we touch the wire _b_ to that post we have, in addition to the primary current which has not voltage enough to produce a spark, a secondary current flowing in the same wire at the same time and having voltage enough to produce a spark. The primary current is continuous while the contact is closed; the secondary current is momentary, as the tongue detects, and is produced only while changes are being made in the strength of the magnetic field. We will now take another piece of wire and wind upon the coil about two hundred more turns, leaving this outer coil wholly disconnected from the inner one, (Fig. 118). I connect _c_ and _d_, the terminals of what we may call the secondary coil, with my measuring instrument and I connect _a_, one of the terminals of the primary coil, with the battery. I then rub _b_, the other primary terminal across the free binding post of the battery. At the instant of closing the primary circuit--that is, of building up the magnetic field--a secondary current is induced in the secondary coil, which lasts for only an instant, too brief a time for the needle to measure it, although its motion indicates both the presence and the direction of the induced current. While the primary circuit remains closed--that is, while no change is occurring in the strength of the magnetic field--the needle returns to zero, indicating no secondary current. But when now the primary circuit is broken and the magnetic field loses its strength, the needle indicates a momentary current in the secondary coil and _in the opposite direction from what it had been at first_. [Illustration: Fig. 118]