286 Mr. A. P. Chattock on the Electrification 



the lines of force upon its sides will balance, and the needle 

 will be urged towards the plate with a force which depends 

 only on the lines ending at its point (its other end being 

 shielded from induction). Moreover, so long as the distance 

 from the point to the plate is greater than a few times the 

 diameter of the point, the distribution of electricity on the 

 latter will be practically independent of that distance ; and 

 the strength of the electrostatic field just in front of the point 

 will thus come to be proportional to the square root of the 

 mechanical pull on the point. The needle becomes in fact its 

 own electrometer, and measures the difference of potential 

 between its point and the air a short distance from it. 



In Table 1. the values of the square root of the pull (P) in 

 dynes on the point of a fine * sewing-needle at the instant of 

 discharge are given for various distances (d) in centimetres 

 between its point and a metal plate. The constancy of VP 

 speaks for itself. 



It is true that an ordinary sewing-needle is not a perfect 

 cylinder, but tapers gradually to its point, so that part of P 

 must be due to lines of force on its sides ; but this must be 

 very small, for the density of charge on the sides is small 

 compared with that at the point, and the force per square 

 centimetre is proportional to the square of the density ; and 

 in addition to this, the force on the sides has to be resolved 

 into a direction almost at right angles to itself before it can 

 affect P. Indeed, the very constancy of P, when d is varied, 

 may be regarded as evidence that this part of P is negligible. 

 (Other reasons are given in § 3.) 



The two needles A and B on which the measurements were 

 made were numbered alike by the makers, and the agreement 

 in the values of VP for the two is fairly close. This is the 

 more satisfactory as the readings were taken under somewhat 

 different conditions. The needle A was suspended in the 

 larger of the two instruments described in § 8. It discharged 

 onto a disk of tin 13 centim. in diameter, with its edge pro- 

 tected by a ring of thick wire. The disk and the inside of 

 the instrument were covered wdth a thin film of vaseline. 

 The readings on B were taken with the small instrument. 

 The disk in this case was a penny with its surface ground and 

 polished. No vaseline was used. 



The positive values of VP for the two needles differ by 

 about 3 per cent., whereas the negative values differ by 10 

 per cent. This is worth pointing out, as it is in accordance 

 with what appears to be a general rule, viz., that the positive 



* Sharp's Egg-eyed needles, No. 10. 



