42 Dr. J. A. Harker on a Direct-reading 



The instrument described represents a form which in sub- 

 sequent work has proved very convenient, and which was 

 desioned and made in the National Physical Laboratory work- 

 shops, after consultation with tlie Director, Dr. Glazebrook, 

 to whom I am indebted for several valuable suffoestions. 



Experience with several forms of Carey-Foster and Cal- 

 lendar-Griffiths resistance-bridges had comdnced me that to 

 calibrate to the necessary accuracy a slide-wire of such size as 

 to make a resistance of 0"1 ohm possible with a moderate 

 length was a difficult piece of work ; and therefore several 

 arrangements were tried with a view of rendering possible 

 the use of a short wire of large cross-section. It is, of course, 

 a considerably easier matter to make a thick wire much more 

 uniform from end to end than a thin one, and the calibration 

 to the accuracy required, especially if the wire be very short, 

 becomes comparatively simple. 



The final arrangement decided upon, after some preliminary 

 experiments on models, I believed at first to be novel ; but on 

 searching the literature 1 find it has been previously used for 

 potentiometers, and is nothing more than a low-resistance 

 form of the Kelvin- Varley slide used in telegraphic work. A 

 diagrammatic representation of the connexions is given in 

 PI. II. fig. 1, and a plan of the top of the instrument in fig. 2. 

 The balancing-coils on which the fall of potential is adjusted 

 to a definite value are in two rows — the centre row of the box 

 being twenty coils of 0*1 ohm each, and in series with these 

 there is a second row immediately behind the slide-wire con- 

 sisting of eleven coils of '01 ohm. Bv means of an arrano-e- 

 ment of thick copper bus-bars connected with the ends of the 

 slide-wire^ which has a total resistance of "02 ohm, any two 

 adjacent coils of this latter series may be put in parallel with 

 the slide-wire. The eleven coils of '01, two of which are thus 

 shunted, are equivalent to exactly '1 ohm. 



For all ordinary thermoelectric work the fall of potential 

 along these two sets of coils is adjusted so that each of the 

 back row represents 1000 microvolts, each "01 being there- 

 fore 100 microvolts. The slide-wire is provided with a scale 

 of 200 divisions figured as 100, which are practicallv milli- 

 metres ; and as the fall along the wire is 100 microvolts, 

 0*1 microvolt can easily be estimated. It will be seen that 

 the slide-wire thus connected acts like a vernier to the small 

 coils. 



The adjustment of the E.M.F. is made by a standard 

 Clark or Weston cell and the auxiliary set of coils in the 

 back row, a feature of the instrument being that without any 

 external alteration either form of standard mav be used at 



