Belas and Hartog — Path of a Small Permeable Body, Sec. 35 



by wetting with glycerine, and tracing the path with a pencil ; but apart 

 from other difficulties, the error of parallax was so great and unequal that 

 we abandoned it. 



Final Method. 



Ultimately the following method of record gave us results freer from 

 error than the experimental conditions to be recorded. 



A strongly built J-plate camera (see Plate I.) was bedded on its side in a 

 teak board, excavated to hold it firmly, and provided with a backward exten- 

 sion carrying leaden weights to prevent overbalancing. Attached to the lens 

 is a right-angled prism, looking directly over the centre of the field, so that it 

 is reflected and focussed on the ground-glass screen of the camera, which is 

 reversed. This permits dots to be made by the " recorder" with a pencil at 

 the successive portions of the screen occupied by the image of the zenith of 

 the spherical float (which is spotted with black) ; and it is these dots that 

 are reproduced in our figures. 



Needless to say, the camera and prism must be so adjusted that the 

 horizontal face of the latter is parallel to that of the fluid, and that the axes 

 of the camera are parallel to those of the trough, so that all perspective 

 distortion is avoided. The prism used was a first-class one by Beck, as was 

 also the objective, a 6" Unifocal Anastigmat, used at full aperture. Both 

 performed admirably. 



A second observer, or rather " controller," is in charge of the exciting 

 current, which must be carefully regulated, since too strong a field would so 

 increase the speed as the float approaches a pole that the observer could not 

 dot in the positions fast enough, and there would also be the danger that 

 acceleration would not be taken up by the friction of the medium. The plate, 

 after completion of the records for a given arrangement of poles (which may 

 take up to eight hours), is now removed, and the dots gone over with water- 

 proof Indian ink. A negative is now made by contact on a backed Imperial 

 Process plate. 



The variation of the intensity of the exciting current is controlled by an 

 electrolytic resistance ; a long china dish containing a sheet of absorbent 

 cotton wool, and well wetted with a dilute solution of sodium phosphate or 

 copper sulphate. The movable electrode is a small block of lead with a 

 short string hanging down, which serves to reduce the current to a 

 minimum without breaking it when the block is raised off the wet cotton. 



An ammeter reading to l/100ths is interposed, for though the motions of 

 the float are the main guide to the controller of the current, the indications 

 of the ammeter enable him after a little while to prevent instead of correcting 

 variations of speed. The source of energy is from the town supply, D.C., 

 230 v. ; the maximum current used about "6 amps., the minimum about - 02. 



f2 



