42 On Magneto-electric Induction in Liquids and Gases. [Mar. 8, 



2. Production of induced current in a mass of liquid rotating over a mag- 

 netic pole. — In this case radial currents should be produced. They were 

 obtained as follows : — Plat porous cells were placed round the circum- 

 ference of a large basin, and in the centre a cylindrical one. These were 

 filled with a solution of cupric sulphate, and contained copper plates. The 

 basin was filled with dilute sulphuric acid. The centre copper plate was 

 connected with one pole of the galvanometer, and the circumferential 

 ones with the other. The whole was placed over the pole of the electro- 

 magnet. On exciting the magnet and rotating the dilute acid, a con- 

 stant current was obtained, flowing from centre to circumference or the 

 reverse according to the direction of rotation. "With platinum electrodes 

 the effect cannot be obtained, but with non-polarizable electrodes it is 

 easily produced. Mercury was likewise tried with still better results. 



3. Production of induced current in a liquid at rest in a variable mag- 

 netic field. — If a flexible tube filled with conducting liquid is wound round 

 an electromagnet, and into the ends electrodes placed so as to include a 

 galvanometer in the circuit, then induced currents are obtained when- 

 ever the strength of the magnet varies. This is the case examined by 

 Faraday. His experiment was repeated by MM. Logeman and Yan 

 Breda (Phil. Mag. [IV.] vol. viii. p. 465), who noticed that the electrodes 

 were left polarized after the induced current had passed. These experi- 

 ments were repeated with more powerful apparatus, using a soft iron 

 wire core within the inducing helix instead of solid iron, and employing 

 a condenser in the primary circuit. Very strong induced currents were 

 obtained, and correspondingly great polarization of the electrodes placed 

 in the ends of the coil of acid. It was hoped that the currents might 

 produce visible inductive electrolysis, but even this improved arrange- 

 ment did not yield that result. Other saline solutions were tried with 

 similar results. 



Lastly, the phenomenon observed by Arago, of the retardation in the 

 vibrations of a magnetic needle oscillated near the surface of liquids, is 

 examined. Evidence is brought forward to show that this is not, as in 

 the case of solid plates, due to induced currents created in the liquid — 

 (1) because the retardation is, ceteris paribus, not proportional to the 

 conductivity of the liquid but dependent on its volatility ; (2) because 

 it takes place equally when a light brass needle, oscillated by torsion, is 

 substituted for the magnet, provided the needle is light and the period of 

 oscillation not very small. Thus a magnetic needle which required 

 4 min. 20 sec. to suffer a decrement of 25° in the semi-arc of vibration 

 when in air, required over dilute sulphuric acid 3 min. 30 sec, over ether 

 2 min. 25 sec. A brass needle of the same dimensions exhibited similar 

 effects. 



In conclusion the magneto-electric induction taking place in moving 

 masses of water on the earth's surface under the influence of terrestrial 

 magnetism is briefly discussed. 



