232 Duncan, Wilkes and Hutchinson — On the 



near it, thus closing the chronograph circuit. The strip was 

 adjustable and the contact was always made as slight as possi- 

 ble consistent with certainty. The length of this contact about 

 -^5- sec, while the clock break was nearly twice this. 



The connection of the axle with the endless screw was made 

 in this way : A small hard rubber screw with square head 

 was fitted in the end of the axle and was joined to the screw of 

 the speed counter by drawing over both a piece of pure rub- 

 ber tubing with thick walls, about 2 cms. long. This connec- 

 tion is easily made, permits no slipping, and absorbs vibrations 

 so completely that even for comparatively high speeds no fas- 

 tening is required to hold the counter down to the board ; but 

 for the very high speeds we used, it was necessary to secure it 

 to the rubber bed on which it lay by 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 cms. diam. and in this work, re- 

 volved in 30 sees.; the length of a second was thus nearly 2 cms.; 

 the sheet could be read with rough means to J mm. (=^0 sec.,) 

 and was actually read much closer. As each observation lasted 

 five minutes, even this gave an estimation of the mean speed 



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 12 sees, after reversing the current 

 through it. 



The resistance "a" in the figure, through which the main 

 current flows, is a large one ohm coil of German silver wound 



about a skeleton cylinder 

 of glass rods, and is about 

 30 cms. high and 15 cms. 

 /eaiv in diameter. The ends 

 of the coil are soldered 

 to copper blocks which 

 form the bottoms of mer- 

 cury cups. It is placed 

 in an earthenware jar fill- 

 ed with a special light oil known to be a good insulator and is 

 provided with a stirring paddle. Resistance U V is a one ohm 

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

 jar and surrounded with water. Resistance "c" is taken from 

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

 same nominal value wound together in a copper cylinder 6 cms. 

 diameter ; they are properly insulated, etc, and protected by a 



