=254. Effects of Electric Currents.=--Having studied some of the devices for producing an electric current, let us now consider some of the _effects_ caused by it. These effects will be studied under three heads: (a) _Magnetic_, (b) _Chemical_, and (c) _Heat_ effects. Devices or articles showing these effects known to most high school students are respectively: (a) the _electromagnet_ (b) _electro-plated silver ware_ and (c) _electric heaters_, such as electric flat irons, electric toasters, etc. The _magnetic_ effect of an electric current was first detected by Oersted at the University of Copenhagen in 1819. It may be observed by holding a wire carrying a current from a voltaic cell above and _parallel_ to the needle of a _magnetoscope_. The needle is at once deflected (Fig. 228). If the current is reversed in direction the magnetoscope needle is deflected in the reverse direction. This simple device is the most common means for detecting an electric current. It therefore constitutes a _galvanoscope_. (See Art. 239.) [Illustration: FIG. 228.--A galvanoscope.] Important Topics 1. Leclanché cells, (a), wet, (b), dry, construction, advantages, uses. 2. Daniell and gravity cells, construction, advantages, uses. 3. Three effects of electric currents, illustrations. 4. The galvanoscope, uses. Exercises 1. Explain how the direction of current in a wire can be determined by a compass. 2. Would you expect to obtain a current from a zinc and copper cell containing a solution of common salt? Perform the experiment. 3. What conditions in a voltaic cell will give a steady electromotive force. 4. What conditions in a voltaic cell will give a strong electromotive force. 5. Name three different electric circuits that you know exist. Which are _open_ and which are _closed_ circuits? 6. Are voltaic cells used in your home? If so, for what purpose are they used? On open or closed circuits? Have you seen them? what kind are they? CHAPTER XII THE MAGNETIC EFFECT OF ELECTRIC CURRENTS. ELECTRICAL MEASUREMENTS (1) THE MAGNETIC EFFECT OF ELECTRIC CURRENTS =255. The Magnetic Effect.=--Of all the effects of electric currents, it is generally conceded that the _magnetic effect_ is the one of _greatest practical importance_, and it is also the one most extensively used. An experiment illustrating this effect has been described in Art. 239. This experiment shows that an electric current, if _parallel_ to a magnetic needle, and _near it_ will deflect the north-seeking pole of the needle to the right or left depending upon the _direction_ of the current flow. This _deflection_ of the magnetic needle is due to the fact that surrounding every electric current are magnetic lines of force. It is this magnetic field of the current that causes the needle to turn. The position taken by the needle is the resultant of the forces of two magnetic fields; one, the earth's field, the other, that of the current.