Current from S. to N. _over_ deflects to W. Current from N. to S. _under_ deflects to W. Current from N. to S. _over_ deflects to E. Current from S. to N. _under_ deflects to E. The Galvanometer, the "Mirror," and "Thomson's reflector" all depend on this principle for their usefulness. These instruments have been fully described in Chapter IV. _The Electro-Magnet._--If a piece of insulated wire be coiled round a rod of soft iron, and a current of electricity be made to pass through the coil, the iron core becomes magnetic as long as the current passes; when the current ceases the magnetism disappears. During the passage of the electric current, the iron core possesses all the properties of a magnet. Therefore if a piece of iron were placed near its poles it would be attracted and released from attraction as often as the current passed or ceased; and supposing such a piece of iron to be retained by a spring, &c., a series of movements, attraction, and drawing back would be effected. A piece of iron so arranged is termed an _armature_, and the instrument is called an _electro-magnet_. The coil of wire must be carefully insulated, or else the electric current will pass through the iron core to earth instead of performing its proper work. An electro-magnet is much more powerful than a steel magnet of equal dimensions, and depends on the strength of the current by which the magnetism is induced, and the number of turns of wire round the core. The north and south poles of an electro-magnet are determined by the direction in which the current flows through the wire. At the _south_ pole the current passes _with_ the hands of a watch, and at the _north_ pole _against_ the hands of a watch. _Definition of the "Ohm."_--The "ohm" is the standard used for electrical resistance; it is obtained by observing what effect is produced by a current of electricity on a certain conductor in a certain time. The ohm is a small coil of German silver wire representing the resistance overcome by a current in a certain time. FOOTNOTES: [Footnote X: Jenkins' 'Electricity.'] APPENDIX.