BALLISTIC GALVANOMETER, a galvanometer (q.v.) designed or used for the measurement of electric currents of very short duration. It does not necessarily differ in any essential particular from other galvanometers, except that the natural period of oscillation of its needle must be long in comparison with the duration of the transient currents that are to be measured. If C is the intensity of the cur rent that is to be measured, and t is the time during which it passes, the general theory of the instrument is as follows: The magnetic moment tending to deflect the needle is propor tional to C, and the angular velocity that such a magnetic moment can produce when acting upon a freely suspended body like the needle is proportional to t. Hence the angular velocity actually communicated to the needle is propor tional both to C and to t; or, in other words, it is proportional to the product of Ct. But an electric "current" (such as is here denoted by C) is defined as the quantity of electricity pass ing per second i and hence Ct is the quantity of electricity passing in the time t. The angular
velocity actually communicated to the needle (which is inferred by observing the extent of the swing) is therefore proportional to the total quantity of electricity passed through the gal vanometer during the short time t, and not to the intensity of the current. This constitutes the chief peculiarity of the instrument. The ballistic galvanometer measures the total quan tity of electricity passed through the instrument, and its readings are in coulombs; while other galvanometers measure the intensity of the cur rent passing, and their readings are in amperes. If the needle of the instrument moves sensibly during the passage of the current, the magnetic movement exerted upon the needle will also vary, even though the current itself remains constant. It is for this reason that the period of free swing of the needle must be long if the instrument is to be used ballistically.