PRINCIPLES OF DESIGN. 



65 



Sometimes it is preferred to express conductivity ia the terms 

 of the mass of the wire instead of its diameter, and the com- 

 pensating device enables the position of balance to be corrected 

 for any small difference of mass between the standard and 

 the test wire of the same length and the same nominal 

 gauge. 



The usual length of wire tested is 30ft., which is unwound 

 from each hank in succession. When the diameter method of 

 measurement is employed these lengths need not be cut off 

 from the hanks, and there is no waste of wire. The wires are 

 extended from the terminals to the conductometer in parallel 

 loops near to each other, so as to be at the same temperature, 

 and they should be supported at the loops by means of two 

 wooden pulleys fixed side by side at such a distance from the 

 instrument as will give exactly 30ft. of wire from terminal to 

 terminal. 



Eeferring to the illustration it will be seen that the contact 

 traverses the bridge wire by a rack and pinion motion, the rack 

 being cut obliquely to give a smooth movement. The fixed 

 mark on the carriage is opposite to the contact of the galvano- 

 meter key with the bridge wire. Immediately above the 

 carriage is the deviation scale, graduated to right and left to 

 2*5 divisions. This can be set with reference to the carriage 

 and then clamped so as to slide with the carriage. Above the 

 deviation scale is the conductivity scale. This is marked 100 

 at the middle and is graduated to right and left, corresponding 

 to a range of conductivity from 95 to 105 per cent. The 

 standard instrument, therefore, measures conductivities from 

 95 per cent, upwards. Before using the instrument the elec- 

 trical centre of the bridge wire must be determined by balancing 

 two equal low resistances on the instrument. A mark upon a 

 small block above the conductivity scale is then set once for all 

 at this point. 



The battery should consist of, say, two 3-pint Gravity Daniell 

 •Cells and the galvanometer should be of low resistance, say, 

 10 ohms. The conductivity scale should then be set with refer- 

 ence to the electrical centre of the bridge at a point corresponding 

 to the known percentage conductivity of the standard wire and 

 it should be clamed in that position so long as that particular 

 standard is used. 



