Lessons in elementary physics / by Balfour Stewart.

  • Balfour Stewart
Date:
1895
Catalogue details

Licence: Public Domain Mark

Credit: Lessons in elementary physics / by Balfour Stewart. Source: Wellcome Collection.

    450/502 page 430
    iron core is thus rapidly magnetised and de-magnetized. The action of this is seen in the diagram. The primary current passes through the adjustable contact at c, and completes the circuit; the soft iron armature is then attracted and the circuit is broken when a spring (not shown in the figure) causes the contact to be again completed. Hence the armature will vibrate backwards and forwards as in the case of the trembling electric bell. When the primary current is started through the secondary coil a number of lines of force pass, and a powerful induced current is therefore generated in the secondary coil, the direc- tion of the induced current being the reverse of that of the primary current. Again, when the primary current is cut off there will be an induced current in the secondary coil, the direction of which will now be the same as that of the primary current. But the secondary current produced when the primary cur- rent is broken has a greater electro-motive force than that produced when the primary current is started, so that when the induced current is forced to overcome a great resistance, such for instance as passing through a space of air, it is only the direct secondary currents, or those produced when the primary current is interrupted, that are able to pass ; and we have therefore virtually, in a RuhmkorfTs coil, a powerful secondary current always in the same direction as the primary current. But when the secondary electrodes are close together alternate currents pass between them. The spark of a very powerful RuhmkorfFs machine has been made to pass through 42 inches of air.' Lesson XLIX.—Distribution and Movement of Electricity in a Voltaic Battery. 415. The Law of Ohm.—This subject was first studied by Ohm, a German philosopher, who developed from theory the laws regulating the motion and distribution of electricity in .a battery. These laws have since been abundantl}' verified by experiment, and may therefore be received as at least a near approximation to the truth. In .a voltaic battery there are three objects of study : first of all we have the electro-niotwe 1 This relates to one made by Mr. Spottiswoode, in which the secondary coil consi.sted of 280 miles of wire.
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