HELMHOLTZ IN BERLIN 



effects of the occlusion of hydrogen on thin plates 

 of palladium and platinum. They found that if a 

 stream of hydrogen is directed to one side of a 

 thin platinum plate by electrolysis, its presence is 

 soon felt on the other side of the plate by the plate 

 becoming more positive, and they showed that what 

 was required by theory was fulfilled by experiment. 



In these investigations Helmholtz made many 

 subsidiary discoveries. Lord Kelvin and he inde- 

 pendently established the reciprocal property of the 

 electric charge of one conductor on another. Thus 

 the potential of a body A due to a unit charge 

 on B is equal to the potential of B due to a unit 

 charge on A. He also invented a double galvano- 

 meter, which was a modification of that of Gaugain. 

 In the instrument of the latter the magnet was 

 suspended, not at the centre of the coil, but at a 

 point on the axis at a distance from the centre equal 

 to half the radius of the coil. Helmholtz greatly 

 improved the instrument, and made it more trust- 

 worthy by placing a second coil, equal to the first, 

 at an equal distance on the other side of the magnet. 1 

 He also constructed an electro-dynamic balance not 

 affected by the magnetism of the earth. 



On April 5, 1881, Helmholtz delivered the 

 Faraday lecture to the Chemical Society On the 

 Modern Development of Faraday's Conception of Elec- 

 tricity, in which he gave an interesting estimate of 



1 Jamin et Bouty, Cours de Physique, t. iv. 2, p. 133. Paris, 1883. 

 211 



