Resistance of the Air to Falling Spheres. 729 



with smoked paper. The fork was connected with one of 

 the secondary terminals of an induction coil, the other 

 terminal being connected with the metallic drum on which 

 the smoked paper was wrapped. The primary circuit of the 

 coil was connected in series with the " dropper " and the 

 " catcher." When the ball was released by the dropper, 

 the primary circuit of the coil was automatically broken for 

 an instant and a spark passed through the paper on the 

 drum making a small puncture. A similar break was made 

 when the ball struck the catcher, and so a similar puncture 

 was produced on the paper. It was thus easy when the 

 paper was unwound to count the number of vibrations of 

 the fork intervening between the beginning and the end 

 of the fall. It was found necessary in practice to have the 

 paper very tightly wrapped on the drum, otherwise the spark 

 sometimes leapt sideways, instead of passing directly from 

 the point of the style. 



The frequency of: the fork was very nearly 125 half- 

 vibrations per second and, as one-tenth of a wave could be 

 estimated with accuracy, the smallest fraction of a second 

 used was 1/625. This was found quite accurate enough for 

 the purpose, as the consistency of time of fall of the same 

 ball for successive falls of the same distance was not great 

 enough to warrant a greater degree of accuracy in the 

 timing. The slight variations in time of fall were probably 

 due to want of perfect sphericity and symmetry in the ball 

 and to movement in the air in the tower. It was found that 

 the best conditions prevailed on dull still days, in spells of 

 cloudy weather. In such conditions the measurement of a 

 time interval of 5 seconds was found to be consistent to about 

 1 in 2000. In clear sunny weather the accuracy was con- 

 siderably less. Consequently favourable opportunities had 

 to be seized when they presented themselves. 



The dropper was made of two laths pivoted at their 

 centres and thus capable of moving somewhat like a pair of 

 scissors. The ball was inverted between the ends of two 

 short pieces of brass tube, one attached to each of the 

 two blades, the distance apart of the tubes being adjustable 

 to fit spheres of different sizes. On pulling a trigger the 

 blades were pulled rapidly apart in a horizontal plane by 

 means of a spring, and the ball was released. A suitable 

 arrangement caused a temporary break in the primary 

 circuit of the induction coil at the same instant. When two 

 balls were used, one was placed between the blades at one 

 end of the dropper, and the other between those at the other 

 end, and the electrical connexion was not used. 



Phil. Mag. S. 6. Vol. 28. No. 167. Nov. 1914. 3 B 



