To store up a certain amount of electric energy in a condenser, we require a certain definite volume of dielectric, no matter how we may arrange it, and the volume required per unit of energy is determined by the dielectric strength of the material. Thus, for instance, ordinary sheet glass cannot be safely employed with a greater electric force than is represented by 20,000 volts for one-tenth of an inch in thickness, or, say, a potential gradient of 160,000 volts per centimetre. This is equivalent to an electric force of about 500 electrostatic units. This may be called the safe-working force. The electrostatic capacity of a condenser formed of two metal surfaces a foot square separated by glass three millimetres in thickness is between 1/360 and 1/400 of a microfarad. If this condenser is charged to 20,000 volts, we have stored up in it half a joule of electric energy, and the volume of the dielectric is 270 cubic centimetres. Hence, to store up in a glass condenser electric energy represented by one joule at a pressure of 20,000 volts, we require 500 cubic centimetres of glass, and it will be found that if we double the pressure and double the thickness of the glass, we still require the same volume.[20] Hence, in the construction of high-tension condensers to store up a given amount of energy, the economical problem is how to obtain the greatest energy-storing capacity for the least money. Glass fulfils this condition better than any other material. Although some materials may have very high dielectric strength, such as paper saturated with various oils, or resins, yet they cannot be used for the purpose of making condensers to yield oscillatory discharges, because the oscillations are damped out of existence too soon by the dielectric. In arranging condensers to attain a given capacity, regard has to be taken of the fact that for a given potential difference there must be a certain total thickness of dielectric, and that if condensers of equal size are being arranged in parallel it adds to their capacity, whilst joining them in series divides their capacity. If N equal condensers or Leyden jars have each a capacity represented by C, and if they are joined _n_ in series and _m_ in parallel, the joint capacity of the whole number is _m_C/_n_, where the product _mn_ = N.