ON GRAPHIC METHODS IN MECHANICAL SCIENCE. 



397 



' Some load diagrams for each montli of the year in the Berlin central 

 stations are given by Professor G. Forbes.' They show the output of the 

 station and the various components in the cost of working. The import- 

 ance of this knowledge is demonstrated by Mr. Crompton,^ and in his 

 paper, and in the report of the subsequent discussion, are a large number 

 of examples of curves rendering observed data more intelligible. From 

 the shape of the curve may be deduced what is the most economical size 

 for the units of the generating plant, and, if direct currents are used, 

 the best combination of dynamos and accumulators. Other useful curves 

 are the variation in the coal consumption, and the variation in the cost 

 per unit of electricity according to the time of year and the nature of 

 the district ; and the latest development in this line is the use of curves 

 in the reports of public companies instead of tables of statistics. These 

 are drawn to show the operations of the year and the increase of 

 business.' 



' The curves of the current used on an electric tramcar or locomotive 

 are very curious, and demonstrate clearly the exceptional nature of the 

 work required of the motor and supply stations, or batteries. Several 

 examples have been given by Mr. Reckenzaun (fig. 25).'' 



Fig. 25. 



Scale- of Ttntey 



ISMin.. 



'A similar set of curves has been prepared, by Mr. A. Siemens, of the 

 power used on the locomotives of the City and South London Electric 

 Railway, presented at the present meeting to the British Association. 



' Curves of the efficiency and candle-power of incandescent and arc 

 lamps have given valuable information on the most economical way of 

 using them, the most usual form being the relation between the candle- 

 power and energy absorbed for different current strengths, and the varia- 

 tion in the quality of the light.'^ 



' Experiments on the output and efficiency of primary and secondary 

 batteries are usually shown in the form of a curve giving the variation in 

 the E.M.F. with respect to time for various current-strengths ^ (fig. 26), 

 and variations of E.M.F. with different strengths of electrolyte. 



• Proe. Inst. E.E., February 1889. ■ Proc. Inst. C. K, April 1891. 



' Report of Second Annual Meeting of City of London Electric Lighting 

 Company. 



* Proc. Inst. C.E., March 1892. 



^ A. P. Trotter on 'The Electric Arc' {Proc. Inst. C.E., April 1892); and see a 

 Report on Glow-lamps {Electrician, July 1892). 



« Ayrton, 'Secondary Batteries' {Proc. Inst. E.E., July 1890); Gladstone and 

 Hibbert, Proc. Inst. E.E., May 1892. 



