A Text-Book of Physics Volume 3

A Text-Book of Physics Volume 3

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This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1912 edition. Excerpt: ... wire and another coil is placed below this on the table (Fig. 30). If currents are sent round both of these in the same direction, they will be equivalent to two small magnets, as the letters on the left-hand side of the figure indicate, and there will clearly be an attraction between them. This may be counterbalanced by placing weights on the other pan. An elaborate instrument on this principle was devised by Lord Kelvin, by means of which the strength of a current can be measured by counterbalancing the attractions between coils by the weights of standard masses. 28. The Electric Bell.--A common application of the electro-magnet is seen in the ordinary electric bell. This is shown diagrammatically in Fig. 31. G is the gong which is hit by a striker S. This FiK. 30. Fig. 31. striker is fastened to the end of a flexible iron ribbon which is fixed firmly at 0. The current from a battery passes through a key K to a pointed platinum wire which touches the iron ribbon at P. The current flows from the ribbon through a flexible wire and then round the coils of a small horseshoe electro-magnet and thence by a wire back to the battery. When the key K is turned so that no current flows there is no attraction between the electro-magnet and the ribbon carrying S. This rilibon therefore remains in the position shown in the diagram. Directly, however, the current is turned on it causes the electromagnet to attract the ribbon and bring the striker sharply against the gong. But when the ribbon moves Fig. 32. Fig. 33. to now. There is, consequently, no attraction between the electromagnet and the ribbon, which will therefore spring back to its old position. This, however, completes the circuit again and the whole series of operations is repeated. As a...show more

Product details

  • Paperback | 72 pages
  • 189 x 246 x 4mm | 145g
  • Rarebooksclub.com
  • Miami Fl, United States
  • English
  • black & white illustrations
  • 1236577590
  • 9781236577597