378 MESSES. V. H. VELEY AND J. J. MANLEY ON 



as to tighten but not twist the wire ; its effective length was 2 metres. As this 

 arrangement obviously required a special form of slider to tap, without sagging the 

 wire, the following plan was adopted : two weighted opposed cones, cc, moved upon 

 two uniform glass cylinders, cy, placed side by side and set parallel to the wire, 

 according to the principle of Lord KELVIN, that a body provided with four points will 

 always move parallel to itself along a cylinder. A brass shoe, s, carrying a glass 

 plate projected from the centre of the cones at a right angle and rested upon the 

 millimetre scale of the bridge ; the position of the slider was indicated by a line 

 etched upon the lower surface of the glass plate, and observed through a little 

 window, iv, situated vertically above it. A platinum contact piece, f, mounted upon 

 an ebonite rod, g, was adjusted to within a fraction of a millimetre of the wire. 

 When it was desired to make contact, an insulated arm, H, was brought down upon 

 the wire from above by tilting the lever, /. In order to move the slider a wheel, wh, 

 Avas attached to one of two pulleys, over which passed a cord kept stretched by a 

 spiral spring. For the purpose of fine adjustment, the wheel, wh, was furnished 

 with a rubber band, cemented at its edge, on which a second and smaller wheel, ivh', 

 could be brought to bear by releasing the lever, J ; from the latter wheel projected 

 an arm, A, which rested upon the end of the milled head screw, S. By this arrange- 

 ment the' slider could be moved by the observer from the extremity of the bridge, 

 and all thenno-currents due to his proximity and direct handling were avoided. 



CALIBRATION OP BRIDGE WIRE. 



The wire was placed in the bridge, stretched to the requisite extent, and allowed 

 to rest undisturbed for nine weeks ; then it was calibrated by CAREY FOSTER'S 

 method, using as a gauge a short length of the same wire soldered on to massive 

 copper terminals and a copper connector of the same length, both of which were 

 joined to the bridge by mercury cups furnished with stout copper tags. The process 

 of calibration was conducted on two different dates. The wire was found to be very 

 uniform throughout, the highest and lowest values for the resistance of 1 millim. 

 being '0001516 and '0001510 ohm respectively. 



The total resistance of the 851 millims. in the centre of the bridge, as also of the 

 ends of the bridge, were determined by a fall of potential method, using one Daniell 

 cell working through a resistance of 200 ohms, a standard coil of '1 ohm value, and 

 a high resistance galvanometer ; the current was uniform within limits of time 

 greatly in excess of those required for the observation ; the necessary corrections for 

 the temperature coefficient were subsequently applied. Ten independent obser- 

 vations were made of the total resistance, one set of two on one date, and another 

 set of eight after an interval of a week. For the interchange of the standard coils 

 and electrolyte, a switch board, constructed upon S. P. THOMPSON'S pattern, was 

 used. The mercury cups were mounted upon ebonite stems to secure better insulation, 



