Practical Cinematography and Its Applications

by Frederick Arthur Ambrose Talbot

Under these circumstances the continuous photographing system is now very extensively employed. It has undergone many wonderful developments and achieved extraordinary success.

One of the most interesting and marvellous of its triumphs was won with the extremely sensitive "string" galvanometer, which was invented by the eminent Dutch scientist, Professor Einthoven. This particular apparatus has been of incalculable value to the medical profession, and Monsieur Lucien Bull has constructed a special camera with the idea of obtaining permanent and continuous cinematographic records of the experiments conducted by means of it.

The apparatus employed for this particular sphere of operations is of a somewhat involved character. Fundamentally the camera is that which was designed by Monsieur Bull for photographing the flight of insects at the rate of two thousand pictures per second, but it has been modified to suit the new conditions. The reason why it offers the best chance of securing a continuous record is that its sensitized ribbon is mounted upon a drum, a single winding of which produces a photographic record about 3 feet 6 inches in length.

The principle of the Einthoven string galvanometer may be described roughly in a few words. There is a very fine conducting wire, or fibre, of platinum or silvered quartz, which is stretched across the magnetic field of the galvanometer. It is extremely thin, being virtually a hair. Now, when an electric current, ever so slight, is transmitted through this fibre, or string, as it is called by the inventor, it is deflected from its position of rest, the extent of the deviation varying with the strength of the electrical disturbance. When a slight current is sent through the string it may betray the fact with no more than a slight tremor, but a stronger current will cause it to move violently.

A pencil of light, from an electric arc lamp, is transmitted through the galvanometer in such a way that the string is brilliantly lighted. An enlarged image of the string is then thrown upon the sensitized ribbon in the camera by means of a powerful microscope lens.

Seeing that the time and distance measurements in such delicate experiments as these are of the first importance, the sensitized surface upon which the record is printed—paper or film—is calibrated photographically while the experiment is proceeding. It is divided into small squares, the longitudinal lines referring to the time factor, while the transverse lines indicate the extent of the movement of the quartz thread.

This continuous record system is of inestimable value in connection with physiological researches when details concerning the beating of the heart are desired. A person who places a finger of each hand upon the extremities of the string, witnesses the recording of his own heart beats. For the brief period between each beat the string remains quiescent in its normal position, and the record of the same, the enlarged shadow thrown by the pencil of light through the microscope lens, is made upon the sensitized surface within the camera in the form of a steady straight line. The beat of the heart sends an impulse of electricity through the galvanometer, and causes the string to deviate rapidly. As the pencil of light is shining continuously through the microscope lens of the instrument, it stands to reason that the slightest tremor of the thread, accentuated in the shadow, must be recorded. No vibration is too slight to be caught. Not only is the extent of the vibration photographed and capable of being calculated by means of the calibration, but, as the sensitized ribbon is travelling continuously past the lens, the duration of the vibration is photographed as well.

In the case of a normal and healthy person the number of vibrations on the record, corresponding to heart-beats, will average about 80 per minute, and their extent or amplitude will remain comparatively even. But if the person is in bad health, excited, or exhausted, the palpitations will be depicted in the most erratic manner, both as regards their occurrence and their force. One very powerful palpitation, for instance, may be followed by a comparatively long interval of quiescence, succeeded by several spasmodic short movements at brief unequal periods.