[Illustration: FIG. 232.--Right-hand rule for a helix.] =258. The Electromagnet.=--These "right-hand" rules are applied in many different devices. Among these, perhaps the most important is the electromagnet, which is used in the electric bell, the telegraph, the telephone, the dynamo, the motor, and many other electric contrivances. The electromagnet is defined as a _mass of iron around which is placed a helix for conducting an electric current_. On account of its large permeability, the iron core of the helix adds greatly to the effectiveness of the electromagnet, since the magnetism of the iron is added to that of the current in the helix. The magnetism remaining in the iron after the current stops is called the _residual_ magnetism. The residual magnetism is small when the core is made of small wires or thin plates, but is larger when the iron core is solid. Like artificial steel magnets, electromagnets are usually of two forms, _bar_ and _horseshoe_. (See Figs. 233 and 234.) For most purposes the horseshoe form is the more effective since it permits a complete iron circuit for the magnetic lines of force. (See Fig. 235.) This is the form used in the electric bell, in the telegraph sounder, and in lifting magnets. (See Fig. 236.) [Illustration: FIG. 233.--A bar electromagnet.] [Illustration: FIG. 234.--A horseshoe electromagnet.] [Illustration: FIG. 235.--A horseshoe electromagnet may have a complete iron circuit for its lines of force.] [Illustration: FIG. 236.--A lifting magnet.] =259. Effective Electromagnets.=--The _magnetic_ effect of a current in a helix is small, hence the force usually is increased by inserting a core of iron. When at first man tried to signal with electromagnets at a distance it was found that the current would not work the electromagnet. An American by the name of Joseph Henry discovered the remedy for this condition. He found that if the copper wire was insulated by wrapping silk thread about it, and then many layers of the silk insulated wire were wound upon a spool with an iron core, that the magnet would work at a great distance from the source of current. If the current is increased, the magnet is stronger than at first. Thus an _electromagnet may be made stronger by_ (a) _increasing the number of turns of wire in its coils_ and by (b) _sending a stronger current through it_. [Illustration: FIG. 237.--A simple telegraph circuit.]