372 Prof. G. C. Foster on Graphical Methods of 



as A P and A' P', in the case of a tangent-galvanometer, will all 

 pass through the same point, and that, in the case of a sine- 

 galvanometer, all lines drawn in the same way as B P and B' P' 

 will be tangents to the same circle. 



These constructions are so simple and can be so quickly 

 made with sufficient accuracy, that by means of them the effect 

 of altering the resistance of a circuit or the number or arrange- 

 ment of the cells of a battery can be exhibited to a class by the 

 help of actual measurements made during a lecture; but in 

 order to make the process still more rapid, I have had an appara- 

 tus made, which may be called " A Galvanometric Slide-Rule," 

 whereby, when two deflections of a galvanometer have been ob- 

 served corresponding to a known difference of resistance, the 

 permanent resistance and electromotive force of a battery can be 

 ascertained in the course of a few seconds. The general arrange- 

 ment of this apparatus is shown in fig. 5, which is drawn to a 

 scale of about -^. M N is a wooden base about 155 centims. 

 long by 15*2 centims. wide, and 4*7 thick, with a groove in the 

 upper surface, of the shape shown in fig. 5a, running from end 

 to end. On the vertical side shown in the figure there is a scale 

 150 centims. long, divided into millimetres and numbered to- 

 wards right and left from zero at a point A 50 centims. from M 

 and 100 centims. from N. Exactly above the zero mark of the 

 scale is a small brass stud, the axis of which passes through the 

 centre of a small graduated quadrant of 12 centims. radius. 

 Another, similar quadrant, A', with a brass stud at the centre is 

 attached to a small board which slides on the upper side of 

 MN and can be clamped by a screw in any position between A 

 and N. There is also a vertical scale 65 centims. long, 

 divided into millimetres and numbered from the bottom upwards, 

 which is fastened to a sliding piece C, whereby it can be clamped 

 in any position between M and A. The faces of the vertical 

 scale and of the quadrants A and A' are flush with the face of 

 the horizontal scale M N. Two thin silk cords, stretched by 

 small weights, are passed over pulleys at P and P', and attached 

 one to each of the pins at the centres of the quadrauts A and A f . 

 The pulleys are so placed that the cords are very nearly in the 

 same vertical plane as the scales M N and C Q. The apparatus 

 is used as follows : — The pulley P is raised or lowered until the 

 cord fixed at A shows, upon the corresponding quadrant, the 

 deflection obtained on the tangent-galvanometer when no extra 

 resistance is added to the circuit ; the sliding piece carrying the 

 quadrant A' is then moved towards right or left so that the 

 number of centimetres in the distance AA' may be the same as 

 the number of units of resistance (or, if more convenient, so 

 that it may be a simple multiple or submultiple of this number) 



