Suppose, for example, that it is required to measure the current between the plates _CD_ (Fig. 21) due to some radio-active material on the plate _C_, for a given difference of potential between _C_ and _D_. At a given instant the connection of the quadrants of the electrometer with the earth is broken. The weight is attached to the quartz plate, and is held in the hand so as to apply the tension gradually. This causes a release of electricity opposite in sign to that given to the plate _D_. The electrometer needle is kept at the position of rest as nearly as possible by adjusting the tension by hand. The tension being fully applied, the moment the needle commences to move steadily from zero is noted. The current between the plates _CD_ is then given by _Q_/_t_ where _t_ is the time of the observation. The value of _Q_ is known from the weight attached. In this method the electrometer is only used as a detector to show that the system is kept at zero potential. No knowledge of the capacity of the insulated system is required. With practice, measurements of the current can be made in this way with rapidity and certainty. Footnote 99: Soddy, _Trans. Chem. Soc._ Vol. 81, p. 860, 1902. Footnote 100: Wilson, _Proc. Roy. Soc._ Vol. 68, p. 152, 1901. Footnote 101: If the apparatus is required to be air-tight, the gold-leaf system can be charged by means of a piece of magnetized steel wire, which is made to touch the rod _R_ by the approach of a magnet. Footnote 102: It is sometimes observed that the motion of the gold-leaf, immediately after charging, is irregular. In many cases, this can be traced to air currents set up in the electroscope in consequence of unsymmetrical heating by the source of light used for illumination. Footnote 103: Wilson, _Proc. Camb. Phil. Soc._ Vol. 12, Part II. 1903. Footnote 104: Walker, _Phil. Mag._ Aug. 1903. Footnote 105: Strutt, _Phil. Trans._ A, p. 507, 1901. Footnote 106: Dolezalek, _Instrumentenkunde_, p. 345, Dec. 1901. Footnote 107: It is very desirable that care should be taken not to release large quantities of the radium emanation inside a laboratory. This emanation has a slow rate of decay and is carried by currents of air throughout the whole building and finally leaves behind an active deposit of very slow rate of change (see chapter XI.). Eve (_Nature_, March 16, 1905) has drawn attention to the difficulty of making refined radio-active measurements under such conditions. Footnote 108: J. J. Thomson, _Phil. Mag._ 46, p. 537, 1898. Footnote 109: Bronson, _Amer. Journ. Science_, Feb. 1905. Footnote 110: J. and P. Curie, _C. R._ 91, pp. 38 and 294, 1880. See also Friedel and J. Curie, _C. R._ 96, pp. 1262 and 1389, 1883, and Lord Kelvin, _Phil. Mag._ 36, pp. 331, 342, 384, 414, 453, 1893. CHAPTER IV. NATURE OF THE RADIATIONS.