Cooling on the Electrical Resistance of Iron. 79 



a b is a platinum -iridium wire of ^ ohm resistance ; R 2 is 

 a resistance of 100 ohms, and R x a set of resistances from -^ 

 ohm to 200 ohms ; the india-rubber-covered connecting-wires, 

 r 1? r 2 , have each a resistance of Jo ohm at the temperature 

 of 17° C; and S is a sliding-piece which, by a suitable spring- 

 and-catch arrangement, can be kept pressed on any part of 

 a b. At the temperature of the room, the resistance of y is 

 nearly equal to that of x, and R a as well as R 2 is nearly 100 

 ohms ; consequently, any slight changes which may occur 

 during the experimenting in the temperature of r u r 2 , and a b 

 do not sensibly affect the balance. The thermometers T! and 

 T 2 register the temperatures of x and y respectively * ; T x has 

 its bulb in the centre of as, and is enclosed by the glass tube 

 G ; this tube is lined with india-rubber at the extremity out- 

 side the air-chamber, so that the thermometer can, if necessary 

 for the purpose of reading, be pulled out to any required ex- 

 tent without causing any cooling ; the glass tube and T x are 

 both slightly slanted upwards to prevent the column of mer- 

 cury from breaking when the temperature is falling. 



For the purpose of preserving the temperature inside con- 

 stant, the ends of the air-chamber are closed by corks, C 1? C 2 , 

 and the vacant spaces inside are stuffed with cotton- wool (not 

 shown in the figure) . The water in the annular space W is 

 heated by a row of burners, made by piercing pin-holes at 

 equal intervals in a copper tube, closed at one end, and con- 

 nected at the other with the gas. By adjusting the supply of 

 gas or by altering the distance of the row of burners from the 

 air-chamber, any required temperature up to 100° C. could be 

 maintained nearly, if not quite, constant for a considerable 

 length of time. In 15 minutes the water could be raised to 

 100° C. ; and then, by diminishing the supply of gas so that 

 the water only just boiled, this temperature could be kept up 

 for 16 hours without adding more water ; when it was neces- 

 sary to maintain the temperature at 100° C. for longer periods 

 of time, the vessel was replenished with boiling water. 



It will be seen from the figure that there is a " Wheatstone's 

 Bridge " arrangement for determining the resistance-ratio, 

 x : y. The temperature of y varied by only a few degrees 

 during the whole of the inquiry ; and as both the actual resist- 



* Only one thermometer was used for T\ in this particular investiga- 

 tion, but the author generally employs, in work of this kind, three ther- 

 mometers in turn. One of these thermometers is graduated from —5° C. 

 to 30° C, the second from 30° 0. to 65° C, and the third from Qo° C. to 

 100° C. ; all three thermometers are divided into tenth degrees, and have 

 been carefully compared with the Kew standards. 



