Strength and Direction of a Varying Electric Current. 103 



and back to the zinc pole of the battery, making a blue mark 

 on the rotating paper just underneath that needle ; while if 

 a current passes through the coil it is deflected to the right 

 or left according to the direction of the current, the circuit- 

 closers on the left or right of the centre successively coming 

 into contact with the column of water between the •capillary 

 plates in order, the result being that the corresponding needles 

 make blue marks on the rotating paper. But since the paper 

 revolves with uniform velocity, it is evident that the longer the 

 time of contact between a circuit- closer and the water between 

 the capillary plates, no matter which circuit-closer it is, the 

 longer the length of the mark on the paper underneath the 

 needle corresponding to that circuit-closer ; and the shorter 

 the shorter. 



From the preceding description it will be clear that when 

 an electric current, varying from time to time in strength 

 and direction, passes through the coil (C), we shall get a curve 

 made up of dots, or of dots and lines, on the moving paper 

 ribbon, the nature of the curve determining the strength and 

 direction of the current at any moment. Fig. 4 shows one 

 of such curves experimentally obtained by allowing a varying 

 current to pass through the coil. Now since the motion of 

 the paper ribbon is uniform, it is easy to find out the point 

 in the curve, or the position of the coil, corresponding to 

 any moment; and since the motion of the coil is non- oscil- 

 latory, each position of the coil corresponds to a certain 

 definite strength of current, which can easily be determined 

 by a simple experiment. So that by an examination of the 

 curve thus obtained, it is easy to find out what was the strength 

 of the current passing through the coil at any moment. 



To give a rough idea of the sensibility of the apparatus, it 

 may be mentioned that when the record shown in fig. 4 was 

 obtained the apparatus was at its ordinary sensibility, which 

 was such, that the superior and inferior limits of the current 

 which it could record were respectively about 4 milliamperes 

 and \ of a milliampere. But of course the sensitiveness of the 

 apparatus can be varied within a considerable range in very 

 much the same way as in Thomson's Siphon Becorder. 



One defect of the instrument, it may be argued, is the fact 

 that it does not record any current which produces such a 

 deflection of the coil that no one of the circuit-closers is in 

 contact with the water between the capillary plates. This 

 defect, however, is not a very serious one, for since the in- 

 strument is intended to be used for recording varying currents 

 which give rise to a curve made up of dots, or of dots and 

 lines, on the moving paper ribbon, it is easy, by examining 



