242 Professor Fleming [June 5, 



being tested. By this means it is easy to verify all the ordinary laws 

 of conduction. We can, for instance, show at once that by cooling 

 an iron wire in iced water its resistance is decreased, whereas in 

 cooling the carbon filament of a glow-lamp its resistance is increased. 

 This method is not generally so convenient as the arrangement 

 first described by Mr. Hunter Christie to the Royal Society in 1833, 

 re-devised ten years later by Wheatstone in 1843, and which has been 

 always curiously misnamed the " Wheatstone's Bridge," even in sj)ite 

 of Wheatstone's own declaration that he did not invent it.* In this 

 arrangement (see Fig. 3) the current from a battery B has two paths 

 open to it by which to complete its circuit, and we employ a galvano- 

 meter with a single coil to discover two points on these two circuits 

 which are at equal potentials. When these two points are connected 

 the galvanometer needle is undisturbed, and it is a simple matter to 

 show that under these circumstances the numerical values of the elec- 

 trical resistances of the two segments A X, X D, of the circuit A D, 

 denoted by P and Q, and the resistances E and S which form the 



Fig. 3. 

 Wheatstone's Bridge arrangement for comparing resistances. 



other branch, are to one another in simple proportion as R is to S — 

 that is, P is to Q as R is to S. In actual work, one form, useful for 

 lecture purposes, which this arrangement takes is that known as the 

 slide wire bridge (see Fig. 4), and which is before you. In this con- 

 struction the battery current flows partly through a uniform wire a h, 

 stretched over a scale, and partly through a standard resistance 11', 

 and the resistance R to be tested placed in series with it. 



We employ a galvanometer G to connect the middle point between 

 R and R' with some point on the slide wire, and we can always find 

 a point on the slide wire such that no current flows through the gal- 

 vanometer. The ratio of the unknown resistance R is to that of the 

 known standard resistance R' in the ratio of the lengths of the two 

 sections into which the contact piece divides the slide wire. Hence 

 R is determined in terms of R'. Another form of this appliance in 

 which all three arms of the bridge consist of coils of wire capable of 



* See Phil. Trans. 1833, Mr. S. Hunter Christie, on the 'Experimental 

 Determination of the Laws of Magneto-Electric Induction.' See also Wheatstone's 

 Scientific Papers, p. 129, 'An Account of several new instruments for determining 

 the Constant of a Voltaic Circuit,' Phil. Trans, vol. cxxxiii. p. 303, 1843. 



