Volt meters and armatures for the alternating current have electro-magnets for their fields as well as for their armatures. Such instruments are equally well adapted for either direct or alternating currents. For when the current reverses its direction it reverses in field and armature alike, and thus a repulsion between like poles is maintained. Such an instrument, however, cannot respond to as slight a current as those previously described, since they must consume some energy in both field and armature. 23. _Telephone Receiver_ (Fig. 61).--It requires a stretch neither of the imagination nor of the truth to call a telephone receiver an electro-magnet, although perhaps it has never been called that before. We took it apart and found that it consisted of a steel-bar magnet _m_ (Fig. 62), with a small spool of wire _w_ around one end of it. The ends of the wire on the spool run along inside the hard rubber shell to the two binding posts _a_ and _b_ at the other end. A disk of sheet iron _S_ is held in the large end of the case very near to, but not quite touching, the end of the magnet. When an alternating current is sent through the wire upon the spool it causes rapid changes in the strength of the magnetic field, if not reversals of the poles of the field, and the iron disk is made to vibrate, keeping time with the alternations of the current. [Illustration: Fig. 61] [Illustration: Fig. 62] In this laboratory we have seen that our current has sixty alternations per second. When it is connected with the receiver the disk, therefore, makes sixty vibrations per second, and produces a tone which has very nearly the pitch of C two octaves below the middle C upon the piano. 24. _Spark Coil_ (Fig. 63).--The automobile spark coil which we have already used is an electro-magnet. The battery sends a current through wire coiled around an iron core. At one end of this iron core is an iron armature which is made to vibrate in precisely the same manner as the armature of an electric bell. This makes and breaks the current and causes rapid changes in the strength of the field. A rapidly changing magnetic field may be used to develop electricity in a conductor, as we have already seen in the case of the dynamo. [Illustration: Fig. 63] How it is used in the automobile spark coil will be shown later. It is sufficient now to mention it as a case of a magnetic field produced by an electric current passing through a wire coiled around an iron core, or, in short, an electro-magnet. Induction coils, Ruhmkorff coils, and transformers, to be described later, are closely related to this. They all create magnetic fields in the same way and are all electro-magnets. [Illustration: Fig. 64. Transformers] VII ELECTRIC HEATING