Electricity for public schools and colleges . he key by which the ciicuit is made and broken. This difficulty is met and overcome by means of the cofide7iserdescribed in § 8. Another obstacle in the way of abrupt bj-eak is the inductionof eddy current in the iron core ; these currents, of course, oppo-ing the cessation of the primary current. This is obviated by B B 370 ELECTRICITY CH. XXII. slitting the core longitudinally or by making it of a bundle of soft iron wires insulated from one another. Again, the iron core takes a certain time to become magnetised and demagnetised. This is not such
Electricity for public schools and colleges . he key by which the ciicuit is made and broken. This difficulty is met and overcome by means of the cofide7iserdescribed in § 8. Another obstacle in the way of abrupt bj-eak is the inductionof eddy current in the iron core ; these currents, of course, oppo-ing the cessation of the primary current. This is obviated by B B 370 ELECTRICITY CH. XXII. slitting the core longitudinally or by making it of a bundle of soft iron wires insulated from one another. Again, the iron core takes a certain time to become magnetised and demagnetised. This is not such an important obstacle ; but even this is nearly entirely obviated by means of the condenser, as will be explained in § 8. Again, so great is the induced in the secondary that there is danger of a spark penetrating the insulating coating of this wire. This danger is obviated by a special method of ^ § 7. Euhmkorffs Coil.—Fig. i. represents a form of coil \Sf—called Ruhmkorffs coil, after the inventor. Here N and P are. Fig. i. the terminals of the primary connected with the battery ; theprimary coil is not shown, but its core A, composed of a bundleof soft iron wires insulated from each other so as to obviate eddycurrents, is seen slightly projecting from the coil; C is an ar-rangement for turning the current off, or on, permanently ; d is of metal called the hammer, which alternately is attracted upto the iron core A, thereby breaking the current, and falls back byits own weight, thus making the current again ; B is the secondarycoil \ p and/ are its terminals ; inside the stand K is the con-denser, whose function will be described later. As a special instance we may quote the dimensions of Spottiswoodeslarge coil. This was constructed on the same principles as the above ;but the arrangement for making the circuit was worked separately,and was not the automatic hammer contrivance shown in the figure. CB. XXII. ARAGOS DISC AND RUHMKORFFS COIL ;7r
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