370 



Sir J. C. Bose and Mr. G. Das. 



therefore very small. I have recently been experimenting with an electric 

 oscillating device, which simplifies the matter still fvirther. Electric current 

 flows intermittently through a coil of wire which sucks in a rod of soft iron 

 attached to the plate-carrier. The force requ.ired for bringing about the 

 oscillatory movement thus acts directly, without any intervention of the 

 eccentric. 



The amplitude of oscillation of the plate is about 3 mm. It is important 

 that the vertical recording plate should be so adjusted that its distance from 

 the recording tip should remain the same during the excursion of the index 

 or during the lateral displacement of the plate moved by clockwork. Failure 

 to secure this makes the dot-marks unequally distinct ; in the worst cases 

 some of the dots may even be missing. The difficulty is obviated by accurate 

 adjustment of the plate in a vertical plane by means of regulating screws. 



With the particular connection and arrangement of levers the recording tip 

 comes down with growth-elongation. It is perhaps more natural to associate 

 upward growth with an up-curve. The record can, however, be easily 

 reversed by turning over the plate and inspecting the dots from the unsmoked 

 side of the plate. 



3. Experimental Adjustments. 



Adjustment of Tension. — I have explained how the tension exerted on the 

 plant could be rendered negligible. In other cases it may be desirable to 

 apply a definite tension or to increase or decrease it in a quantitative manner. 

 Normally, the levers are so adjusted by suitable counterpoise, as to bring the 

 recording tip to the zero position, which is half-way up the plate. A weight 

 of, say, 1 grm. is now placed on the attachment A (fig. 1) and a weight, T 

 (found by experiment), placed on a fixed notch in L, so as to bring the 

 recording tip once more to zero position. It is clear that when the plant is 

 attached to A, T exerts on it a tension of 1 grm. Various weights are in a 

 similar manner employed to exert tension on the plant from 1 to 10 grm. The 

 tension generally used is about 3 grm. 



Determination of the Magnifying Poiver. — There are two micrometric 

 adjustments, S and S' (fig. 3), one rough and the other fine, by which the 

 plant may be raised or lowered and the recording tip brought to any part of 

 the glass plate. With the connection of the levers shown in fig. 1, the 

 upward growth of the plant lowers the right arm of the lever L, and the 

 recording tip of the second lever L' also moves downwards with the growth 

 of the plant. To determine the magnifying power, a fine black dot is made 

 at the point of attachment of lever and plant, and a particular division of 

 the scale of a microscope-micrometer is focussed to the mark. By means of 

 the micrometer screw, S, a fixed rod attached to the first lever is lowered till 



