270 Proceedings, Sc,^ for 1886. 



These difficulties may be almost entirely obviated by placing the 

 spiral in a stream of running water. This enables a much smaller 

 wire to be used, and it is not so liable to variations of temperature ; 

 at any rate, they are not sudden. In this way I have put the full 

 load on a 7000 Watt machine doing 300 amperes, and had all the 

 resistance in a trough 4 feet long by 8 feet square. 



Large electrolytic cells have also been used, with carbon or lead for 

 the electrodes, but the change of resistance is rapid, and there is also 

 the counter E.M.F. to be allowed for. 



If pure water is used as the electrolyte, its resistance with 

 electrodes of any convenient size is very high, too high indeed — unless 

 the cell be made inconveniently large — for any E.M.F. that is not 

 considerably over 100 volts. Two carbon rods, for instance, 6 in. 

 X ^ in. diameter, placed one inch apart in ordinary Yan Yean water, 

 have, roughly, a resistance of 50,000 co, which on a 50 volt 

 machine would only represent a load equal to yoVo of a 20 c.p. 

 Swan lamp. 



If sulphuric acid or a saline solution is used, its composition 

 changes very rapidly, and the temperature rises so much that it is 

 desirable to have a running stream, which either alters the strength 

 of the solution, or necessitates a very large reservoir of the liquid. 



Having then selected a form of resistance, the circuits 

 may be arranged as in Fig. 1. There it will be seen that the 

 current and E.M.F. are measured directly, and the work done is 

 determined. 



In the case of machines intended for lighting purposes it is often 

 desirable to see the lamps actually alight; at any rate, it is often 

 likely to satisfy an intending purchaser better than the readings on 

 some delicate and mysterious instrument which he does not 

 understand. In that case lamps may be introduced as in Fig. 2, if 

 the machine is to burn them in simple parallel. 



When the E.M.F. of the dynamo is much higher than that of a 

 single lamp they may be arranged as in Fig. 3, where the effect 

 of both the current and the E.M.F. may be seen in the different 

 groups of lamps, and the E.M.F. may also be checked by a low 

 reading volt-meter placed in parallel with the lamps. 



With the circuit arranged as in Fig. 1 we have, in order to put 

 any given load on a dynamo, to so adjust the resistance that we get 

 the required current, while the E.M.F. is maintained at the desired 

 voltage by modifying the speed of the engine. With a high tension 

 dynamo, as depicted in Fig. 3, we have to adjust the resistance till 

 the volt-meters Vi and V2 read the same, and then the load can be 

 determined by the reading of the am-meter A and volt-meter Vs, or 

 the E.M.F. may be obtained by taking the fall of potential due to 

 one lamp, indicated by Yi and Y2, and multiplying it by the 

 number of lamps in series. 



