NOTES AND APPENDICES. 33 



Dynamo-electric machines, considered as a source of elec- 

 tricity, cannot, by reason of the numerous conditions of their 

 action, be classed with other electric sources, such as liquid 

 batteries or thermo-electric batteries, and therefore, thanks 

 to the kind co-operation of Robert Gray, the engineer of the 

 Indiarubbcr Works Co., we undertook at Silvertown, in the 

 month of July, 1879, a series of experiments to establish, 

 apart from all theoretical considerations, the electric elements 

 of dynamo-electric machines placed under certain conditions 

 of action. 



Our experiments had reference to Gramme machines of 

 the A pattern, called the workshop pattern (represented on 

 page 75 of this volume.) We must mention that similar ex- 

 periments had been already made in France by Mascart and 

 Angot (Journal de Physique, 1878), and in England by 

 Hopkinson (Institution of Mechanical Engineers"). The 

 former were undertaken more particularly from a theoretical 

 point of view, from which we cannot here regard them ; the 

 latter relates to Siemens' machines, and it is a similar inves- 

 tigation that we desire to have made in France on the 

 machines which are here most in use. 



Dynamo-electric machines are set up to work at a given 

 speed, which it is convenient to maintain in order to produce 

 from these machines all they can yield without damaging 

 their parts or the solidity of their construction. We suppose, 

 then, that the speed of rotation is constant, and we refer the 

 electric elements to a normal velocity of 1,000 turns per 

 minute. When the register indicates a greater or less velocity 

 it is always easy to reduce it to this standard, for it is found 

 by experiment that, other things being equal, the electro- 

 motive force is proportional to the number of revolutions of the 

 machine even for variations reaching to 300 turns a minute. 



We were particularly desirous of experimentally explaining 

 the variations of the electric elements of the Gramme 

 machines, by causing the external resistance to vary from 10 

 ohms to an external resistance of nothing. 



