Electricity for public schools and colleges . B, E3 fallsto zero and Ea is deflected with +, and so on. (ii.) A Leyden jar (see next section) is mounted on an it is being charged the outer coating is put to earth, and then tlie wholeis left insulated. W^e can, by alternately touching the knob connected with the inside, andthe outside coating, draw off sparks of + and — electricity in turn?. Thesigns of these alternate charges can readily be tested as in Chapter IV. § 7. Leyden Jars.—When it is required merely to store a largecharge at the potential of the source, and whe
Electricity for public schools and colleges . B, E3 fallsto zero and Ea is deflected with +, and so on. (ii.) A Leyden jar (see next section) is mounted on an it is being charged the outer coating is put to earth, and then tlie wholeis left insulated. W^e can, by alternately touching the knob connected with the inside, andthe outside coating, draw off sparks of + and — electricity in turn?. Thesigns of these alternate charges can readily be tested as in Chapter IV. § 7. Leyden Jars.—When it is required merely to store a largecharge at the potential of the source, and when there is no needto alter the distance between the plates, the condenser usuallytakes the form of a jar of glass having inside and outside coatingsof tin-foil. A knob gives connection with the inside of the jar ;this knob should, if possible, pass up from the inside without anyconnection with the neck of the jar, in order to insure goodinsulation. Since the charge, caterisparibus., is proportional to the differ- 86 ELECTRICITY CH. ence of potential of outside and inside, it should be our objectto make this difference as large as possible. As a rule the highestavailable potential is that of our source, and with this we connectthe knob ; the lowest available is the zero potential of the earth, and so we put the outside coating to earth. A larger charge would be obtained if we could put the outside to a — potential. When the outer coating is to earth, the charge of the jar is determined by the formula Q=K. V where K is the capacity of the jar, and V isthe potential of the inner coating. Glass is used for several reasons. It iseasy to make glass jaxs; glass has a highspecific inductive capacity {see Chapter IX.);and the jar can be made thin, or the distancebetween the plates small and therefore thecapacity of the jar large, without a dischargetaking place through the the calculation of K, as well as for the units in which Vis measured, the reader is referred
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