_C_ = 50, _d_ = 5, _D_ = 1000; then _i_ = 2·8 × 10⁻¹³ amperes. Since the electrometer can readily measure a current corresponding to a movement of half a scale division per second, we see that an electrometer can measure a current of 3 × 10⁻¹⁴ amperes, which is considerably below the range of the most sensitive galvanometer. The capacity of the electrometer itself must not be considered as equal to that of the pair of quadrants and the needle when in a position of rest. The actual capacity is very much larger than this, on account of the motion of the charged needle. Suppose, for example, that the needle is charged to a high negative potential, and kept at the zero position by an external constraint. If a quantity _Q_ of positive electricity is given to the electrometer and its connections, the whole system is raised to a potential _V_, such that _Q_ = _CV_, where _C_ is the capacity of the system. When however the needle is allowed to move, it is attracted into the charged pair of quadrants. This corresponds to the introduction of a negatively charged body between the quadrants, and in consequence the potential of the system is lowered to _V´_. The actual capacity _C´_ of the system when the needle moves is thus greater than _C_, and is given by _C´V´_ = _CV_. Thus the capacity of the electrometer is not a constant, but depends on the potential of the needle, _i.e._ on the sensibility of the electrometer. An interesting result of practical importance follows from the variation of the capacity of the electrometer with the potential of the needle. If the external capacity attached to the electrometer is small compared with that of the electrometer itself, the rate of movement of the needle for a constant current is, in some cases, independent of the sensibility. An electrometer may be used for several days or even weeks to give nearly equal deflections for a constant current, without recharging the needle, although its potential has been steadily falling during the interval. In such a case the decrease in sensibility is nearly proportional to the decrease in capacity of the electrometer, so that the deflection for a given current is only slightly altered. The theory of this action has been given by J. J. Thomson[108]. =68.= The capacity of the electrometer and its connections cannot be measured by any of the commutator methods used for the determination of small capacities, for in such cases the needle does not move, and the capacity measured is not that of the electrometer system when in actual use. The value of the capacity may, however, be determined by the method of mixtures. Let _C_ = capacity of electrometer and connections, _C₁_ = capacity of a standard condenser.