100 Messrs. Duncan, Wilkes, and Hutchinson on the Value 



rubber bands, in order to ensure perfectly uniform contact 

 between the pin and the spring. 



The chronograph was a large and excellent instrument by 

 Fauth ; the drum was about 18 centim. diam., and in this 

 work revolved in 30 seconds ; the length of a second was thus 

 nearly 2 centim. ; the sheet could be read with rough means 

 to J millim. ( = 4 L ( j sec); and was actually read much closer. 

 As each observation lasted five minutes, even this gave an 

 estimation of the mean speed to goV o- 



The galvanometer was a low resistance one of the Thomson 

 reflecting type ; a small piece of wire which dipped in a light 

 oil was hung from the needle and acted as a damper : with 

 this the needle was found to be sufficiently sensitive, and to 

 come nearly to rest in about twelve seconds after reversing 

 the current through it. The resistance a in the figure through 



to gaZv 



which the main current flows is a large 1-ohm coil of German 

 silver wound about a skeleton cylinder of glass rods, and is 

 about 30 centim. high and 15 centim. in diameter. The ends 

 of the coil are soldered to copper blocks which form the bot- 

 toms of mercury-cups. It is placed in an earthenware jar 

 filled with special light oil known to be a good insulator, and 

 is provided with a stirring-paddle. Resistance b is a 1-ohm 

 coil, by Elliot, of the usual form ; this is put in a large glass 

 jar and surrounded with water. Resistance c is taken from 

 specially made " comparators ; " each consists of ten coils of 

 the same nominal value wound together on a copper cylinder 

 6 centim. diameter ; they are properly insulated &c, and pro- 

 tected by a larger concentric cylinder. The terminals are 

 soldered to the copper bottoms of mercury-cups arranged in 

 two circles around the hard rubber ring which closes the 

 annular space between the inner and outer copper cylinders. 

 The inner cylinder is filled with water. The connexions of 

 the ten coils can be varied at pleasure ; they can all be thrown 

 in series, in parallel, or in any intermediate arrangement. 

 There were two comparators used, with the coils 100 and 10 

 ohms respectively. 



