166 Prof. Ayrton and Mr. Hay craft on an Apparatus for 



vessel used to hold the liquid is a thin glass beaker of just 

 sufficient diameter to take the framework of manganin strip. 



Electric connexion with the two stiff wires which form 

 the handle is made by means of two well-insulated very 

 flexible leads L L, each composed of a strand of about 210 

 copper wires *011 inch diameter. The size of the stout 

 copper wires and of the flexible lead was chosen with the 

 intention that there should neither be heat received nor heat 

 lost on account of these connexions. The sectional area of 

 the insulated flexible lead is rather greater than that of the 

 stout copper wires, and the ultimate rise of temperature of 

 both with a current of 30 amperes is about 7° C; the rise of 

 temperature during the short time that the current is passed 

 being thus about equal to that of the water, and automatically 

 preventing gain or loss of heat. The number of watts taken 

 by the copper wires and flexible leads at 30 amperes is 

 about 1*9. 



It may here be pointed out that it is far better, from the 

 point of view of getting a good mechanical design, to use a 

 large current at a low pressure than to take the same power 

 from a small current at a considerable pressure. Thus it is 

 easy to design a strong stirrer of the shape described above 

 which will have a resistance of ^ of an ohm, carrying 30 

 amperes at 10 volts pressure; but it would be by no means 

 so easy to make an equally efficient and substantial stirrer of 

 33*3 ohms resistance to take 3 amperes only at 100 volts. 



It was thought worth while to ascertain to what extent 

 conduction through these leads influenced the rate of coolino- 

 of the vessel when no current was passing, and cooling curves 

 were therefore taken with the manganin framework in the 

 vessel and attached to the leads. The average rate of cooling 

 was found to be 0*000242 calorie per square centimetre area 

 of surface per 1° C. excess temperature; which is only about 

 4 per cent, greater than in the previously mentioned experi- 

 ments, in which a light wooden stirrer only was used, the 

 value then obtained being 0'000232. 



III. Use of tlie Apparatus. 



In the following table are given the results of several 

 successive experiments made with the apparatus described 

 above by Messrs. Solomon and Grogan, students at the Central 

 Technical College, under our supervision. The amperes and 

 volts are expressed in the international units adopted at the 

 Chicago Congress. The ammeter and voltmeter used "were 

 the well-known Weston-d'Arsonval type instruments. The 

 current was known within about ^ of one per cent., and the 



