Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: CHAPTER III CHARGE AND INDUCTION 15. Condensers.—Returning to the subject of charging bodies electrically, how is one to consider the fact that bringing an earth-plate near a conductor increases its capacity so greatly, enabling the same pressure to force in a much larger quantity of fluid ? how is one to think of a condenser, or Leyden jar ? In the easiest possible way, by observing that the bringing near an earth-connected conductor is really thinning down the dielectric on all sides of the body. The thin-walled elastic medium of course takes less force to distend it a given amount than a thick mass of the same stuff took ; in other words, a cavity enclosed by thin walls has much more capacity than if its walls were thick. Remember that capacity of elastic cavities cannot satisfactorily be measured as the capacity of buckets is measured, by the maximum quantity they will hold when full: they are never " full," till they burst ; and the amount requiredto burst them measures rather their strength than their capacity. The only reasonable definition of capacity in such cases is the ratio which any addition to their contents bears to the extra pressure required to force it in : and this is exactly the way electrical " capacity " is denned. A Lcyden jar is like a cavity with quite thin walls—in other words, it is like an elastic bag. But if you thin it too far, or strain it too much, the elastic membrane may burst: exactly, and this is the disruptive discharge of a jar, and is accompanied by a spark. Sometimes it is the solid dielectric which breaks down permanently. Ordinarily it is merely the air; and, since a fluid insulator constitutes a self-mending partition, it is instantaneously as good as new again. There are many things of interest and importance to study a...