610 ELECTRICAL ENGINEERING 



steam engine would eventually be superseded by the rotary steam 

 engine. Conversely, this improvement in steam engineering is re- 

 acting to the benefit of electrical engineering by reducing the size 

 and cost of steam dynamos and the price of electric power in dis- 

 tributing-systems. 



The development of any one branch of engineering inevitably 

 stimulates other branches. The reduction in the cost of electric 

 power for machine-driving thus promotes activity in the construc- 

 tion of all kinds of machinery. 



The development of electrical engineering has also tended to in- 

 crease the accuracy and precision of other branches of engineer- 

 ing; first, by simplifying the delivery and measurement of power, 

 and second, by the introduction into engineering of a scientific 

 system of units. 



Mechanical power is delivered from one body or system to another 

 through mechanical contact, or the pressure of one material sys- 

 tem upon its neighbor. In general, the power transmitted is equal 

 to the product of the effective pressure, or tension, and the velocity 

 at which it is delivered. In most cases it is very difficult to deter- 

 mine the magnitude of the effective pressure. In the electric trans- 

 mission of power, the power is delivered through an electric con- 

 ducting circuit, and while in the circuit is equal to the product of 

 a certain voltage and a certain current strength. This product 

 the electric power is readily capable of precise measurement. 

 Consequently, the most convenient and accurate method of meas- 

 uring the delivery of mechanical power is usually by electrical means 

 through the intervention of an electric circuit. Thus the power 

 which a machine receives from moment to moment in the perform- 

 ance of its duty, or the total energy which it receives in the course 

 of a given period of time, may be determined with great conven- 

 ience and a high degree of commercial accuracy by electrical meas- 

 uring-instruments placed in the circuit of a motor coupled to the 

 machine. By the accumulation of such observations and expe- 

 rience the knowledge of the behavior of machinery has been greatly 

 augmented since the general introduction of dynamos and mo- 

 tors. 



It is a remarkable fact that in spite of our lack of knowledge 

 as to the precise fundamental nature of electricity and magnetism, 

 the knowledge of their action and control should already be so 

 definite and precise. In many instances it is possible to design and 

 predetermine the behavior of electric machines as closely as it is pos- 

 sible to determine their behavior experimentally after being built, 

 under commercial or factory conditions. That is to say, a skilled 

 designer, accustomed to a certain class of dynamo machines, can 

 frequently compute the characteristic properties of a new dynamo 



