INDUCTION 



2973 



INDUCTIVE METHOD 



INDUCTION, induk'shun, ELECTRIC. Any 

 conductor of electricity may be electrified by 

 placing it in an electrical field, that is, by 

 placing it near an electrified body. This effect 

 may be shown very simply by electrifying a 

 rod of sealing wax or vulcanite by rubbing it 

 with a woolen cloth and then placing the rod 

 near two metal balls which are in contact on 

 a glass plate, which is an insulator. The metal 

 balls then become electrically charged. The 

 ball that is nearer to the negative charge on 

 the sealing wax is positively charged; the ball 

 that is farther from the rod is negatively 

 charged. If the balls are separated while the 

 rod is near them each ball holds its charge. 

 A piece of sheet rubber (a non-conductor) may 

 be used in moving the balls in order not to 

 conduct away the charge through the hand. 

 The electroscope (which see) can be used to 

 prove that one ball has a positive and the 

 other a negative charge. The process illus- 

 trated by this experiment is electric induction. 



If a positive charge is brought near an elec- 

 tric conductor the conductor has two equal 

 charges induced upon it, one positive, the 

 other negative. The negative charge is on 

 the part of the conductor nearest to the posi- 

 tive charge which induces it. See ELECTRICITY; 

 INDUCTION COIL. 



INDUCTION COIL, a device for increasing 

 the strength of an electric current. It consists 

 of an inner coil of coarse wire a, and called 

 the primary; a core b, made of soft iron 

 wires; an outer coil c, made of fine insulated 



d d 



INDUCTION COIL 



The figure is explained in the text. 



wire and called the secondary; two points or 

 knobs d d and a vibrating apparatus e j called 

 the interrupter, for making and breaking the 

 current that is sent through the inner coil. 

 When the points ej are brought in contact 

 a positive current enters, flows through the 

 inner coil and back to the battery. The spring 



that holds these points together has a piece 

 of soft iron on its upper end directly opposite 

 the end of the core 6. When the circuit from 

 the battery is closed, this core becomes an 

 electromagnet and draws the iron towards it, 

 opening the circuit at e. The core then loses 

 its magnetism and the spring pulls the piece 

 of iron back, forming the connection with the 

 battery again, closing the circuit and starting 

 the whole process over again. The circuit 

 is opened and closed with great rapidity. 



The wire in the secondary coil is very long, 

 and the rapid opening and closing of the cir- 

 cuit in the primary coil induces in it a current 

 of much greater force than that in the primary 

 coil. In the largest induction coil ever made, 

 the secondary coil was 280 miles in length and 

 was wrapped about the primary coil in 340,000 

 turns. When operated with a battery of thirty 

 cells, it caused a spark forty-two inches long 

 to leap between the opposite ends of the coil. 

 The strength of the coil is measured by the 

 distance between the points d d across which 

 a spark will pass. 



The chief uses of induction coils are for 

 producing X-rays and in wireless telegraphy. 

 See ELECTROMAGNETISM ; ROENTGEN RAYS; 

 WIRELESS TELEGRAPH. 



INDUCTIVE, induk'tiv, METHOD, that 

 method of instruction which proceeds from 

 individual ideas gained through observation to 

 the formation of definitions, classifications and 

 rules. It is also called the method of observa- 

 tion and the method of discovery. At first 

 children gain their knowledge by observation 

 and experience; hence all children learn in- 

 ductively. By comparison of ideas they gradu- 

 ally form general notions. The child first gains 

 an idea of his own dog, for instance, then by 

 observation of other dogs he forms the idea 

 dog. This idea does not mean merely his dog, 

 but any dog, that is, it is a class or general 

 idea. In a similar manner children learn the 

 principles of number and many of the defini- 

 tions in geography. The inductive method is 

 especially adapted to instruction in the pri- 

 mary, grades; it is also the method employed 

 in scientific discovery, and it is valuable in 

 beginning the study of most new subjects. 



The inductive method, however, should not 

 be carried too far. The child should be led 

 to apply his knowledge as fast as he gains it, 

 and to proceed by the inductive method in 

 gaining every new idea would be a waste of 

 time. In the study of algebra, geometry or 

 grammar, for instance, much more can be ac- 



