H. A. Rowland et al. — Ratio of the Units of Electricity. 289 



while the line of zero load has the equation 



h 



h + P'= - 



(h + P e + E+(b+c)n} . . (27) 



as is readily determined from (22). 



The writer has examined engines in which the friction pres- 

 sure increases with the pressure of the supply steam at constant 

 load. The value of J\ then becomes P Q -\-en, and the surface 

 of brake horse-power as a function of mean effective pressure 

 is then represented by equations similar to that which in this 

 paper represent brake horse-power as a function of boiler pres- 

 sure. The discussion then becomes more complex, although it 

 can be made on the lines here laid down. It is better to avoid 

 this discussion by refraining from building such engines. 



The experience of the writer with con den sing-engines has 

 been very limited, but it would appear that the equations here 

 given will apply also to them. 



The four surfaces here discussed may all be constructed by 

 means of threads to represent the two sets of rectilinear ele- 

 ments in each. These are constant speed, and constant load. 

 Such models represent the working conditions of an engine in 

 a most interesting way. 



Art. XLI. — On the Ratio of the Electromagnetic to the Elec- 

 trostatic Unit of Electricity ; by Henry A. Rowland, 

 with the assistance of E. H. Hall and L. B. Fletcher. 



The determination described below was made in the labora- 

 tory of the Johns Hopkins University about ten years ago, 

 and was laid aside for further experiment before publication. 

 The time never arrived to complete it, and I now seize the 

 opportunity of the publication of a determination of the ratio 

 by Mr. Rosa in which the same standard condenser was used, 

 to publish it. Mr. Rosa has used the method of getting the 

 ratio in terms of a resistance. Ten years ago the absolute 

 resistance of a wire was a very uncertain quantity and, there- 

 fore, I adopted the method of measuring a quantity of elec- 

 tricity electrostatically and then, by passing it through a 

 galvanometer, measuring it electromagnetically. 



The method consisted, then, in charging a standard condenser, 



whose geometrical form was accurately known, to a given 



potential as measured by a very accurate absolute electrometer, 



and then passing it through a galvanometer whose constant 



was accurately known, and measuring the swing of the needle. 



Am. Jour. Sol— Third Series, Vol. XXXVIII, No. 226.— October, 1889. 

 19 



