The geo-electric response will then afford us a measure of the intensity of excitation induced at various angles of inclination. The mechanical response on account of its inherent defects does not afford us the true relation between the angle of inclination and intensity of geotropic reaction. But the electric method of inquiry is free from the defects of the mechanical method.
Experiment 176.—The specimen was rotated so that the angle of rotation was 45°, and the maximum electric response observed. The angle was next increased to 90° and the reading for the enhanced response taken. The ratio of the geo-electric response at 90° and 45°, thus affords us a measure of the effective stimulations at the two angles. I give below a table which gives results obtained with 24 different specimens.
TABLE XXXV.—RELATION BETWEEN ANGLE OF INCLINATION AND GEOTROPIC EFFECT.
| No. of specimen. | Galvanometric deflection. | Ratio b⁄a. | |
| (a) at 45° | (b) at 90° | ||
| 1 | 70 divisions | 110 divisions | 1·5 |
| 2 | 30 " | 45 " | 1·5 |
| 3 | 90 " | 126 " | 1·4 |
| 4 | 70 " | 100 " | 1·4 |
| 5 | 21 " | 33 " | 1·6 |
| 6 | 30 " | 50 " | 1·6 |
| 7 | 12 " | 20 " | 1·6 |
| 8 | 14 " | 20 " | 1·4 |
| 9 | 10 " | 16 " | 1·6 |
| 10 | 45 " | 75 " | 1·5 |
| 11 | 25 " | 40 " | 1·6 |
| 12 | 14 " | 20 " | 1·4 |
| 13 | 13 " | 20 " | 1·5 |
| 14 | 30 " | 50 " | 1·5 |
| 15 | 38 " | 54 " | 1·4 |
| 16 | 50 " | 75 " | 1·5 |
| 17 | 55 " | 90 " | 1·5 |
| 18 | 13 " | 20 " | 1·5 |
| 19 | 17 " | 25 " | 1·4 |
| 20 | 80 " | 130 " | 1·5 |
| 21 | 15 " | 22 " | 1·4 |
| 22 | 45 " | 75 " | 1·5 |
| 23 | 135 " | 220 " | 1·6 |
| 24 | 55 " | 93 " | 1·5 |
| Mean ratio = 1·49 | |||
The mean ratio 1·49 may thus be regarded as the relative geotropic effects at 90° and 45°; this is practically the same as Sin 90°⁄Sin 45° = 1·4. Hence we arrive at the following law:
The intensity on geotropic action varies as the sine of the directive angle.
METHOD OF VERTICAL ROTATION.
I have hitherto described results obtained with the Method of Axial Rotation; I shall now take up the second method, that of Vertical Rotation, diagrammatic representation of which is given in figure 166V. The specimen is held vertical and two electrical contacts, A and B, made with the two lateral sides; it is then rotated round a horizontal axis perpendicular to the length of the specimen. Rotation may be carried in a right-handed direction with increasing angle with the vertical. The point A is thus subjected to enhanced geotropic stimulation and exhibits increasing electric change of galvanometric negativity; continuous decrease of angle of inclination to zero by rotation in the reverse direction causes a disappearance of the induced electric change. The rotation is next continued in the negative direction by which the point B is increasingly subjected to geotropic action. B is now found to exhibit excitatory reaction, the current of response having undergone a reversal. Rotation to the right and left will be distinguished by plus and minus signs.
ELECTRIC RESPONSE THROUGH AN ENTIRE CYCLE.
Experiment 177.—When the specimen is vigorous, characteristic response with its changing sign may be obtained through an entire cycle from 0° to +45° to +90°; then back to 45° to 0° to -45° to -90°. With less vigorous specimens the responses becomes enfeebled under fatigue. I give below the results of a typical experiment carried out with a vigorous specimen, the response being distinguished as - when A is above, and + when A is below, the inversion bringing about a reversal direction of the responsive current.