214 Mr. F. W. Burstall on the Use of 



and the wire can be tightened by nuts on the ends of the 

 rods. Thus the wire is not soldered to the terminals, and can 

 be removed for repair. The scale is divided into 1000 parts, 

 and the divisions are then divided into 10 by estimation ; 

 this is easily done with the aid of a good magnifier. The 

 slider moves on the scale itself, and is provided with a fine 

 adjustment. The connexion to the galvanometer is made by 

 a platinum- tipped spring, resting on a platinum wire which 

 is laid parallel to the main wire. The equal coils are made 

 of manganin, and have a resistance of about 10 ohms. They 

 have several times been adjusted to exact equality, but have 

 never preserved that equality for a long period. This in- 

 equality, however, is unimportant, since it is eliminated by 

 double balancing, which has been done at least once during 

 each set of determinations. The actual amount of inequality 

 at the present time is shown by the facts that, with 1 ohm in 

 each of the outer arms, the bridge centre (/. e. the mean of 

 the readings when the coils are interchanged) is 500*25, 

 whilst with 1 ohm in each arm it is 500*4, and with 10 ohms 

 in each arm it is 500*5. 



The wire is mounted on a stout marble base. To protect 

 the wire as much as possible from heat radiated from the 

 observer's body two blocks of wood have been hollowed in 

 such a manner that they can be slid over the wire ; the slider 

 itself being wrapped in cotton- wool. These precautions re- 

 duce the effect of the thermo-currents to a few scale-divisions 

 on the galvanometer. The effect of thermo-currents on the 

 galvanometer was eliminated by the galvanometer-circuit being- 

 kept closed, the battery-circuit being made and broken as 

 required. In order to connect the box-coils and the standards 

 to the bridge, three pairs of heavy flexible copper leads have 

 been constructed ; one pair has mercury- cups, intended for 

 the reception of the standard coils, at one extremity ; a 

 second pair has conical plugs, so that any particular coil in 

 the box can be measured alone ; the third pair and the other 

 six extremities of the leads are provided with heavy copper 

 spades, by means of which connexion can be made with the 

 terminals of the box and the bridge. The three pairs of leads 

 are adjusted to exact equality one with another, and each has 

 a resistance of '00966 ohms at a temperature of 11°*8. The 

 values of these resistances do not enter into any determination 

 as the leads were always in pairs, one in each arm of the 

 bridge. 



The galvanometer is a four-coil astatic instrument, the 

 resistance of each coil being 60 ohms. The magnet system 

 is supported by a fine quartz fibre 260 millim. long. In 



