.s/A' WILLIAM SIEMENS, F.R.S. 41! 



batteries may be charged occasionally by the dynamo on the car 

 in running down hill, or they may be charged in running upon 

 Irvrl ground from the dynamo at the power station. In like 

 manner for boat propulsion, the secondary battery is the only 

 available means ; because we could never hope to attach a vessel 

 permanently to a rope connected with the shore. The batteries, 

 although heavy, may be made part of the keel-weight ; and 

 although neither with the railway nor with the boat should 1 

 rxptvt long distances to be traversed with electricity as a motive 

 power, for short distances, and under conditions where steam 

 power is for various reasons not applicable, I believe we have in 

 electric energy an efficient and practicable means of propulsion. 



I had intended to say something on the subject of electrical 

 units, and also of the principles involved in measuring currents ; 

 but this is a subject which Sir William Thomson will bring before 

 you in a much more complete and exhaustive manner than I can 

 hope to do. I hold, however, in order to know anything about a 

 physical effect, you must be able to measure it, and it is therefore 

 impossible to deal with electric quantities without at the same 

 time looking round at the means at our hand for measuring and 

 weighing, as it were, one effect against the other. The Electric 

 Congress, which met in Paris in 1881, laid down certain general 

 rules, and made certain determinate units for the use of the elec- 

 trician the Ohm, the Volt, the Ampere, the Coulomb, and the 

 Farad. But there seemed to be a general want felt for a unit 

 that would give us more directly the amount of work done by a 

 given current ; and last summer, in delivering my presidential 

 address to the British Association, I ventured to propose two 

 additional units the Joule, representing the unit of heat or of 

 work accomplished by a unit of current in a unit of resistance ; 

 and the Watt, the unit of power, or the Ampere flowing through 

 the Volt. This proposal, I am glad to find, has met with very 

 general acceptance. We now measure the power of the dynamo 

 machine in Watts ; and the advantage of this measurement of 

 Volt-Amperes, or Watts, is, that it is the best expression of the 

 power of a machine of given dimensions, which will be capable of 

 producing the same Watt power, either of a high potential and 

 small quantity when thin wire is used on its coils, or of low 

 potential and larger quantity when thick wire is used. Seven 



