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ON STANDARDS OF ELECTRICAL RESISTANCE. 435 



thickness as to give the coil a time of vibration such that it completes a period 

 in about thirteen seconds. The upper end of the suspending wire is attached 

 to a milled head, and this head can be turned round by the fingers. The lower 

 end of the wire is firmly fixed to the coil, and is in metallic connexion with 

 one end of it. To the other end of the coil is soldered a spiral of very fine 

 platinum wire which hangs directly below the coil, and its lower end is 

 cemented to the dry woodwork of the instrument. To the fixed end of the 

 spiral coil a copper wire is attached, whose other end is soldered to a bind- 

 ing-screw in an accessible position. 



On one side of the small or moveable coil is fixed a plane mirror, and in 

 front of the mirror, at a distance of about 450 centims., the scale is fixed on 

 which the observations are read. A pai'affin-lamp wire, to give dark line in 

 image of flame, and lens are used in the ordinary way for finding accurately 

 the angle through which the coil turns. It is never greater than "OS, Its 

 true amount can be determined to within -jL per cent. 



The connexions are not very intricate, and are traced thus : — Starting 

 from one pole of the battery (the battery used was sixty sawdust Daniell's 

 in series), the current goes in at one end of large coil No. 1, and from the 

 other end of No. 1 the current goes to either end of the moveable coil, and 

 the end of the moveable coil at which we suppose the current to be coming 

 out is connected with the end of No. 2 large coU, similar to the end of No. 1, 

 to which the battery was first attached, that is to say, the end which will 

 make the current go round in the same direction in both the large coUs. 

 When the current leaves the extreme end of No. 2, it passes though a 10,000 

 B.A. resistance-box ; the current is completed by connecting the other end 

 of the resistance-box with the pole of the battery not already engaged. 



The absolute electrometer is used in the ordinary way for measuring dif- 

 ferences of potential, and its electrodes are connected, one to the end of the. 

 dynamometer coil No. 1, which is joined to the battery, and the other elec- 

 trode is fixed to the end of the resistance-box, which is connected to the 

 other pole of the battery. Thus the greatest diff'erence of potential in the 

 arrangement is measured by the absolute electrometer. An electrometer 

 key is used to reverse these connexions in the course of the experiments. 



There is only one other part of the arrangement to be explained, and that 

 is the method of observing the resistance of the dynamometer coils while the 

 experiments are going on. This was done by means of the resistance-box in 

 the circuit and an electrometer. At one time the standard electrometer was 

 used for this purpose, biit more lately the quadrant, rendered unsensitive, 

 was employed. Both these instruments are described in the last Report. 



To take the resistance of the coils, the electrodes of the electrometer were 

 first i^laced on the extreme ends of the three coils, and the difference of po- 

 tential was ascertained. The electrodes were then shifted to the ends of the 

 resistance-box, and the difference of potentials of its two ends was found. 

 This gives at once the resistance of the coils. 



There are two things which have to be done before the experiments are 

 commenced. One is the determination of the moment of inertia of the 

 moveable coil. This is done at the beginning and end of a long series of 

 experiments, by comparing it with a ring whose moment of inertia is known. 

 The other is done every day, and it is finding the time of vibration of the 

 small coil after all the connexions have been made, and the coil put into its 

 place. This was done both with the current from the battery flowing 

 through the coils and with no current flowing ; but this variation was of 

 very little conseqiience, as no difference could be detected in the time. When 



