A solenoid was wound on a small glass tube, sixty turns of wire, seven turns to the centimetre. A leyden-jar, charged up to a convenient potential by a Voss machine, discharged through this solenoid, and any iron, whether solid or finely divided, placed inside the solenoid was always more or less magnetized by the discharge. Plate XLVIII., Fig. 1. C. ordinary leyden-jar; A is solenoid; S, spark-gap. The whole of the discharge passed through solenoid A. After the discharge had passed the needles were examined by means of a small mirror magnetometer. As this magnetometer is used in all future experiments for testing the magnetization of needles, the construction is briefly explained. It was made on the pattern set forth in Gray's “Absolute Measurements,” vol. ii., p. 79. The needle was small, and arranged in a cavity, so that it was nearly dead-beat. The deflection was increased by means of a lamp and scale in the ordinary way. The value of the horizontal component at the needle was 0.22, and remained practically constant, as there were no masses of iron in the vicinity. It was first settled that the needle placed in the solenoid was unaffected by the charging current from the Voss. The Voss was turned so as to charge up the jars just below the potential necessary to spark across knobs at S. The needle was then removed and tested by the magnetometer. No effect was observed. The effect of discharges on needles of different diameters was first investigated. Length of needles, 7cm.:— (1.) Part of steel knitting-needle, diameter 0–103in.: Deflection 112, at 9cm. distance from magnetometer needle. (2.) Pianoforte steel wire, diameter 0–032in.: Deflection, 40; distance from magnetometer, 9cm. (3.) Thin steel wire, diameter 0–008in.: Deflection, 10; distance from magnetometer, 9cm. Diameter. Deflection. 0.103in. 112 0.032in. 40 0.008in. 10 It will be observed from these experiments that the deflection is very nearly proportional to the diameter of the wire. This is to be expected, as the magnetizing forces are confined to a thin skin of the substance. The amount of the magnetization of the wire is proportional to the surface of the iron, and not to its sectional area, as it is for steady currents. In order to show that the effect was a surface one, and did not