PEIllCE. — CORRECTION FOR COUNTER ELECTROMOTIVE FORCE. 165 



stops Oil the inductor rod were used, and in each of them throws were 

 observed when the rheostat resistance was 0, 50, and 70 ; the total 

 resistance of the galvanometer circuit outside the rheostat was 60.8. 

 For the first position the throws were comparatively large (196.3, 

 112.2, and 95.7), and the apparent resistance of the galvanometer 

 circuit was 66.6 besides the rheostat resistance. For the other setting 

 of the stops, the throws were much smaller (42.1, 24.6; 20.95), and the 

 apparent circuit resistance 70.2 ohms. In some galvanometers the ap- 

 parent resistance, when calculated from large throws as measured on 

 the calibrated scale, will be larger than when deduced from small 

 throws. In other galvanometers the opposite rule holds. 



(G) A galvanometer of the same design as the one mentioned in (F), 

 but with many more turns in the coil and a resistance of 730 units, 

 was tested with the "magneto-inductor." This fine instrument, which 

 is furnished with a strong permanent magnet, has been used in making 

 a great number of measurements of permeability, and the very great 

 difference between the apparent and the real resistance (770.3) of the 

 circuit outside of the rheostat, illustrates strikingly the danger of 

 neglecting the correction for the counter electromotive force induced 

 in the coil during a swing. For one setting of the stops on the in- 

 ductor coil, the readings caused by the galvanometer throw were 66.2 

 or 39.7, according as the rheostat resistance was or 1000; these 

 numbers point to a circuit resistance of 1498 units. For the second 

 setting of the stops, the readings were 88.2 and 52.9, and these cor- 

 respond to an apparent circuit resistance of 1499 units. The apparent 

 resistance is in this case nearly double the real resistance. For wide 

 ranges of throws the apparent resistance of any circuit in which this 

 galvanometer may be placed remains almost exactly constant, and the 

 instrument is wholly trustworthy when absolute measurements are to 

 be made, if the apparent resistance instead of the true resistance be 

 used. The apparent resistance of this galvanometer is naturally not 

 quite the same in all circuits ; when the electromotive force to be in- 

 duced in a coil attached to it is larger, it is of course necessary to 

 introduce a large extra resistance into the circuit, and this seems to 

 reduce very slightly the difference between the apparent and the real 

 resistance of the circuit; but so far as my experience goes, it has 

 always been about 700 units. A condenser discharge of one micro- 

 coulomb sent through this galvanometer gives a scale reading of 

 about 100 mm. 



(H) A very useful instrument for hysteresis work on iron cores of 

 electro-magnets is the microamperemeter of Paul, which has the form of 

 an ordinary portable commercial amperemeter, and is of moderate cost. 



