CHRON'OSCOPE chronoscopc. It. ero noseopo, from Gk.xp6vor. chronos, time -I- °work .ekopus, watcher, from aKopeic,skopein, to watch). An instrument for measuring aoeurately small intervals of time, used in psychology, physiology. and in many departnu.nts of experimental and applied physies. The first attempt to measure a very small period of time was made by Sir Charles Wheatstf me in IS3.4. when he endeavored to ascertain the duration of the electric spark. This was accomplished by inealis of a revolving mirror; and as this apparatus was the first and simplest of II number of similar instruments, it may lie described at some length. (hi a ho•i zontal axis eapald t` of being revolved at high speed. a mirror was fixed. The rays issuing from a luminous point. such as a small flame, were reflected from the mirror when at rest, and readied the eye of tie (observer. If the mirror is slowly revolved, the image of the bambinos point will be either raised or lowered, and if the speed of rotation is increased, instead of 11 of images there will be seen a continuous streak or band of light. If the given source of light be supplanted by an electric spark, and the mir ror set in rotation, as long as the duration of the spark is less than is required by the mirror to reil•et the rays across the field in the form of a baud, then the spark will appear as a point; but, increasing the speed of the mirror, a stage Mill be reached where the iniage will lie produced in the form of a bright streak. The time of the spark can then be calculated in the following way: First the velocity of the mirror is observed; then, suppose that the image of the spark ex tends over what is equivalent to one-half a de gree of arc. As the movement of the reflected ray is, from the laws of reflection, twice that of the reflecting mirror, it. would follow that when the mirror was revolving at it velocity of SOO revolutions per second, the lime consumed by the mirror in causing the spark to appear as a streak of light would be X X ,h-x 4 or of a second.
Wheatstone also employed this apparatus to study the time consumed by an electric current in passing over a conductor, using for this pur pose the sparks furnished at different lengths of the conductor, and measuring the difference in time between their occurrences. A rotating mir
ror was also used by Feddersen in his researches on the electric spark, and again by Rood, the former employing a concave mirror, while the latter used a plane mirror in connection with a system of lenses, To Wheatstone is due another form of ehrono scope. which was used for measuring the veloc ity of a projectile from the time it left a can non until any desired point was reached. This apparatus in its essence consisted of a clock which was set in motion and stopped by the armature of an electromagnet. Across the muzzle of the gun a wire was placed which was broken at the exit of the projectile and the circuit opened. thus releasing the armature of the electromagnet and setting in motion the clock work. When the shot reached the required dis tance the circuit was closed. and the clock stopped in a similar fashion. Wheatstone did not claim a greater accuracy for this instrument than to of a second, but improvements by Hipp, who, keeping the clockwork in motion. used the magnets only to throw the indicating part of the apparatus into gear with the move ment, made the instrument more serviceable.
W. Siemens used a rapidly rotating cylinder, on a paper covering of which sparks front a cir (-nit containing 'Leyden jars made the record inarks. The introduction of a seconds pendulinn into the circuit gave a record of the time by causing spark-punctures at regular intervals. and the desired times could be found by com paring the distances. With this apparatus the velocity of a projectile while in the bore of the vanilla' could be measured, it being. ()Illy nary to insert insulated conductors at different points where the circuit could be made or broken by the traveling shot. Siemens also used this form of chronoseope, or chronograph, to measure the velocity of the electric current. Helmholtz improved the instrument by providing mecha nism to give a constantly increasing velocity. making his measurement when the desired rate of speed was readied, the regulation being effect fd by the action of centrifugal force. This form of instrument has been employed extensively in physiological work, and there are numerous modifications now in use.
Tuning-forks (see TUNING-FORK; CIIRONO