PHYSICS. 403 



the electromotive force corresponding to the energy gained in the 

 net chemical change ensuing. By taking suitable precautions in the 

 construction of a cell, one may be obtained the electromotive force of 

 which does not difler more than ± 0.25 per cent, from 1.114 volts. 

 But such a cell cannot be kept for many hours without altering in 

 value materially, and is in practice, therefore, a far less convenient 

 standard than the Clark mercurous sulphate cell. {Nature, February, 

 XXV, p. 403 ; Phil. Mag., April, Y, xiii, p. 265.) 



Potier has given a description of the various classes of dynamoelectric 

 machines exhibited at the Electrical Exposition in Paris in 1881. The 

 Gramme, Edison, Siemens, Burgin, Giilcher, and Brush machines are 

 figured. The ring of the Brush machine is criticised as giving rise to 

 injurious Foucault currents ; so that while the Gramme and the Siemi^ns 

 machines give above 90 pei; cent, efl&ciency, that of the Brush machine 

 is below 80. {J. Phys., September, II, i, p. 389.) 



Lacoine has investigated the increased resistance in dynamo-electric 

 machines due to increased velocity of rotation, and finds it to be due to 

 variations in contact between the commutator and brushes. Using a 

 copper cylinder 5<'™ in diameter, having longitudinal grooves cut in its 

 surface, two steel springs were made to press on opposite sides of it, 

 which were in circuit with a battery, telephone, and galvanometer. 

 When at rest, the resistance of the circuit was 68 ohms; at 2,000 rota- 

 tions, 183; at 4,000, 900; at about 5,000, 1,567; at a higher velocity 

 not measured, 2,900. Each spring touched the cylinder over three 

 grooves, and increasing the pressure diminishes the resistance. Using 

 a smooth cylinder, the results were the same except that a little higher 

 velocity was necessary to obtain the same resistance. {Comptes Eend^is, 

 xciii, p. 958; Phil. Mag., January, V, xiii, p. 76.) 



Sir William Thomson has devised two dynamos, one gi%ing continuous, 

 the other alternating currents. In the former, a drum-armature in the 

 form of a barrel is used, the staves being of copper and insulated from 

 one another. At one end of the drum these copper bars are all united 

 to one metallic plate, and at the other their prolongations serve as com- 

 mutator bars. Inside this armature is a stationary electro-magnet, its 

 poles facing those of the nearly circular field magnets placed outside. 

 The armature is supported on friction rollers, the lower pair made of 

 non-conducting material, the upper of copjjer, to take off the currents. 

 The alternating current machine is a disk dynamo, in which the rota- 

 ting armature has no iron. The disk is made of wood, having projecting 

 wooden teeth upon its sides, around which the copper wire or strip is 

 carried alternately backwards and forwards, going finally to the axle. 

 The field magnets are placed round the circumference of a circular 

 frame with their poles alternating and facing inward. They are formed 

 of a cast-iron ring with lugs screwed ujDon its face, around which, back- 

 ward and forward, the zigzag conductors pass. {Nature, November, 

 xxvn, pp. 58, 78.) 



