The method, based on the propagation of modulated light waves, was developed by E. Bergestrand of the Swedish Geographical Survey in collaboration with the manufacturer, M/s AGA of Sweden. Of the several models of the geodimeter manufactured by them, model 2-A can be used only for observations made at night while model-4 can be used for limited daytime observations.
Fig. shows the schematic diagram of the geodimeter. Fig. shows the photograph of the front panel of the model-4 geodimeter mounted on the tripod. The ‘main instrument is stationed at one end of the line (to be measured) with its back facing the other end of the line, while a reflector (consisting either of a spherical mirror or a reflex prism system) is placed at the other end of the line.
The light from an incandescent lamp-
(1) is focused on utilizing an achromatic condenser and passed through a Kerr cell
(2). The Kerr cell consists of two closely spaced conducting plates, the space between which is filled with nitrobenzene. When high voltage is applied to the plates of the cell and a ray of light is focused on it, the ray is split into two parts, each moving with a different velocity. Two Nicol’s prisms
(3) are placed on either side of the Kerr cell. The light leaving the first Nicol’s prisms is plane polarised. The light is split into two (having a phase difference) by the Kerr cell. On leaving the Kerr cell, the light is recombined. However, because of phase difference, the resulting beam elliptically polarised Diverging light from the second polariser can be focused to a parallel beam by the transmitter objective, and can then be reflected from a minor lens to a large spherical concave mirror.
(4)On the other end of the line being measured is the put a reflex prism system or a spherical mirror, which reflects the beam of light to the geodimeter. The receiver system of the geodimeter consists of the spherical concave mirror, mirror lens, and receiver objective. The light of variable intensity after reflection impinges on the cathode of the phototube.
(5). In the phototube, the light photons impinge on the cathode causing a few primary electrons to leave and travel, accelerated by a high-frequency voltage, to the first dynode, where the secondary emission takes place. This is repeated through a further eight dynodes. The final electron current at the anode is some hundreds of thousand times greater than that at the cathode.
(6) The sensitivity of the phototube is varied by applying the high frequency-Kerr cell voltage between the cathode and the first dynode. The low-frequency vibrations are eliminated by a series of electrical chokes and condensers.
The passages of this modulating voltage through the instrument are delayed utilizing an adjustable electrical delay unit.
The difference between the currents during the positive and negative bias period is measured on the null indicator which is a sensitive D.C. moving coil micro-ammeter.
To make both the negative and positive current intensities equal (i.e. to obtain null-point), the phase of the high-frequency voltage from, the Kerr cell must be adjusted ± 90• concerning the voltage generated by light. at the cathode.
(7), Thus, the light which is focused on a narrow beam from the geodimeter stationed at one end to the reflector stationed at the other end of the line has reflected the photomultiplier.
The variation in the intensity of this reflected light causes the current from the photomultiplier to vary where the current is already being varied by the direct signal from the crystal-controlled oscillator.
The phase difference between the two pulses received by the cell is a measure of the distance between the geodimeter and the reflector (i.e., length of the line). The distance can be measured at different frequencies.
On Model·2A of the geodimeter, three frequencies are available. Model-4 has four frequencies. Four phase positions are available on the phase position indicator. Changing phase indicates that the polarity of the Kerr cell terminals of high and low tension is reversed in turn.
The ‘fine’ and ‘coarse’ delay switches control the setting of the electrical delay between the Kerr cell and the photomultiplier. The power required is obtained from a mobile gasoline generator.
Model-4 has a night ·range of 15 meters to 15 km, a daylight range of 15 to 800 meters, and an average error of ± 10 mm ± five millionths of the distance. It weighs about 36 kg without the generator.
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