LIQUIDS AND ALLIED EXPERIMENTS. 



41 



mr 



will not last indefinitely. In other experiments the hose ii was branched 

 and led to a glass reservoir, showing the water level on the U-tube principle. 

 In such a case the water level w could be read off at the reservoir, by a cathe- 

 tometer or some equivalent simpler attachment. 



In the latest form of apparatus the guard-ring cup holding the water at w 

 was dispensed with and replaced by the prolonged tube g (now made of glass) 

 as shown in fig. iib. Here gg is the glass tube 0.75 to 1 inch in diameter 

 communicating with the hose i below and holding the float / submerged as 

 far as the water level w. The advantage of this form is this, that all parts 

 of the floating tube may be seen and the floating level read off at v, for 

 instance. It is necessary, however, to keep the tube in the middle of the 

 water level, at w, by aid of three horizontal 

 adjustment screws (not shown), at a hori- 

 zontal angle of 120 , surrounding the tube 

 ff very loosely. In such a case the disk e 

 is kept centered, the tube / is never in 

 contact with gg, and a little tapping obvi- 

 ates all danger of friction. 



Fig. iib also shows the level controlled 

 by a tubular glass receiver hh with its 

 water level visible at w'. This is to be 

 held on a vertical slide micrometer, so 

 that the shift of hh, due to a charge on cc, 

 may be accurately read. In fact, this form 

 of instrument, requiring no subsidiary 

 apparatus, was finally adopted throughout. 

 The vertical slide h need only be a few 

 centimeters long and provided with a moving vernier for a fixed scale. 



For potentials higher than 20,000 or 30,000 volts, the vertical height of 

 the space cd must be much increased, to prevent the continuous rise of the 

 disk ee. Hence the guard ring should be much larger than in the figure or 

 the disk appreciably smaller. Furthermore, in this case the sharp screw 

 55 and similar sharp edges elsewhere as in the disks cc, dd, etc., are inadmis- 

 sible. Without rounded edges and a rounded screw, the secondary dis- 

 turbances due to electric winds interfere with the interpretation of the 

 measurements and the apparatus will not take a high potential. All parts 

 except the disk cc are of course put to earth, and possible induction between 

 cc and other conductors except dd must be scrupulously guarded against. 



Finally, fig. 12 shows an alternative float consisting of the disk e corre- 

 sponding to fig. 1 1 A, the tube ff passing the preferably conical capsule pp 

 about 1.5 cm. high and 5 cm. in diameter, of thin brass hermetically sealed. 

 The tube ff is prolonged below by the solid brass rod n or sinker. When 

 placed in the cup, fig. 1 1 a, or in a similar vessel, the water level is at ww 

 and may be adjusted by dropping small weights down the tube ff. The 

 same method, fig. 13, is used for placing this level. The whole arrangement 



Fig. 12 — Hydro- 

 meter and disk 

 carried by air- 

 chamber. 



Fig. 13. — Com- 

 pression screw 

 for adjusting 

 levels. 



