392 Reports on Special Researches 



mulator battery and charging system. The use of prunary batteries renders it necessary 

 to economize on the running of the motors and so the active deposit has been collected for 

 only half an hour each day. 



For charging the central system of the collecting apparatus a means of obtaining a 

 potential of about 2,000 to 4,000 volts is necessary. Zamboni piles are not suitable for such 

 work at sea, for they fall very considerably in potential unless the insulation can be main- 

 tained to a high degree in all parts of the apparatus. A form of Kelvin water-dropper 

 multiplier has been found most convenient for the work in hand. Such an apparatus 

 possesses the advantage that it is not permanently injured by short circuits or by subjection 

 to faulty insulation for long periods. In the shnplest form of water-dropper a tank A, 

 Figure 25 (to the left), supplies water and forms a jet in a cylinder B, which is insulated and 

 connected to one pole of a battery C, whose other pole is earthed. Under these conditions 

 the funnel D and its attachments become charged by the falling drops, and in practice the 

 potential of D rises until the rate of electrical leakage over insulating material, etc., equals 

 the rate of supply of elecricity by the drops. The latter quantity can be increased by 

 increasing the potential of the battery C, but to this there is naturally a practical limit. 

 Very Uttle is gained by simply increasing the number of jets in the cylinder, for the effect 

 obtained is proportional to the electrical capacity of the droplets, and this quantity 

 increases very slowly with increase of the number of jets when the latter are close together. 

 If, however, the cyUnder B is divided into a number of separate compartments, as in Figure 

 25 (to the right), and one jet is allowed to break in each compartment, the effect of the jets 

 should be additive, and an apparatus of this kind was consequently designed for use on 

 the Carnegie. Some tests carried out on this apparatus, by S. J. Mauchly, showed that 

 the contributions of the jets were strictly additive, and the apparatus with 7 jets and a 

 potential of 100 volts on the cylindrical compartments gave, on short circuit, a current of 

 105 e.s.u. 



High potentials and relatively large currents can also be obtained by using Lord 

 Kelvin's double multiplying system; but, for the particular work in hand, it was felt that 

 the above method was much more suitable. 



The air-flow is required in cubic centuneters per second, but the anemometer used 

 with the apparatus reads in linear feet. Apart from this, however, the indications of an 

 anemometer, when used in a tube as in the present experiment, are not related in a sunple 

 way to its indications in the open. It was consequently necessary to determine a reduction 

 factor k, to reduce the apparent indications of the instnmaent to cubic centimeters per 

 second. 



To this end was constructed a dummy apparatus which, as far as essentials were 

 concerned, was of the same geometrical form as the main apparatus. At the end remote 

 from the anemometer were inserted three wire grids, of such a size as to extend right over 

 the cross-section of the cylinder. The central grid, which was of manganin, served as an 

 electrical heater, and the other two grids, which were of copper, functioned as resistance 

 thermometers, and were used differentially. The energy supplied to the central grid was 

 measured when the steady state had been attained, and this quantity, combined with a 

 knowledge of the specific heat of air and the temperature rise between the outer grids, 

 served to give the au--flow in absolute units. It was thus possible to calibrate the anemom- 

 eter under the exact conditions in which it was used. The actual observations concerned 

 in the determination of the factor k were made by D. M. Wise. 



The observations are recorded as in the example shown on page 400. The quantity 

 T] represents the number of pairs of ions produced per second in the ionization chamber, due 

 to the active material which would be deposited in an air-flow of 1 c. c. per second, r] is 

 recorded for various values of the time estimated from the completion of the deposition, 

 and serves as a preliminary^ quantity for use in the subsequent determination of the radium- 



