Page:Electricity (1912) Kapp.djvu/86

82 In the old method of making condensers with paper as a dielectric the coatings or "electrodes" were sheets of tinfoil, but in the modern type of condenser developed by Mr. Mansbridge, of the British Post Office, so-called metallised paper is used interleaved with plain paper, both being paraffined. The effect of this improvement is that the bulk, weight and cost of paper condensers have been reduced to less than one tenth of what they were formerly. The capacity of any condenser is given in electrostatic c.g.s. units by the formula

$$C = K \frac{S}{4\pi \delta}$$

This unit is inconveniently small, and for practical work a much larger unit, namely, the "microfarad," has been adopted. As the name implies, the microfarad is the one millionth part of the farad, and the magnitude of the farad is given by the following definition: A condenser of one farad capacity, when charged under the e.m.f. of one volt, will store that quantity of electricity which is represented by the flow of one ampere during one second. The ratio of the electrostatic unit of capacity to the microfarad is $$1$$ to $$9 \times 10^5$$, so that the capacity of a