1G2 PROCEEDINGS OF THE AMERICAN ACADEMY. 



relative resistances of the galvanometer and the rest of the circuit, the 

 moment of inertia of the coil, and the magnitude of the coefficients of 

 the air and electromagnetic damping. Two galvanometers which have 

 the same ballistic sensitiveness to a given discharge from a condenser, 

 and equal resistances, may, of course, act very differently so far as the 

 effect of the induced counter electromotive force is concerned when 

 placed in inductive circuits which outside of themselves are alike. It 

 is useless, therefore, to attempt to predict with any great accuracy the 

 magnitude of the disturbance due to the induced current in any par- 

 ticular instance, but it may be worth while to give briefly the general 

 results of observations in some actual cases, by way of illustration. 



(A) A certain d'Arsonval ballistic galvanometer, such that a con- 

 denser discharge of one microcoulomb sent through it caused a throw 

 corresponding to a scale reading of about 11.0 cms., was connected in 

 series with a Du Bois and Rubens " Panzer Galvanometer " (P) and 

 with one of two coils of fine insulated wire which formed single layers, 

 the one on a wooden core 2.38 cms. in diameter, the other upon a rod of 

 soft iron 0.94 cms. in diameter. Either coil could be placed within a 

 solenoid of 22.3 turns per centimeter through which a small steady 

 current from a storage cell could be sent, and reversed at pleasure by 

 aid of a commutator in the circuit. By means of a clamp, operated 

 from without the case of the d'Arsonval galvanometer, its coil could be 

 held firmly fixed in such a position that the flux through it of the field 

 of its permanent magnet was practically zero, and it could be released 

 again at will. A heavy suspended system was used in P in order to 

 make the sensitiveness about the same as that of the other instrument 

 when the time of swing was a little shorter than that of the moving 

 coil. The whole resistance of the galvanometer circuit was about 

 2000 ohms. The slightest movement of the coil of the d'Arsonval galva- 

 nometer was sufficient to set the needle of the other instrument in 

 motion also, and the reversal of a given current in the solenoid always 

 caused a larger throw in P when the moving coil was clamped than 

 when it was free. In a typical case the throw of P's needle as meas- 

 ured on the scale of the instrument was 83.5 mm. when the suspended 

 coil was held still, but only 69.9 mm. when it was released. Each of 

 these numbers is the mean of a number of closely accordant readings. 

 Whether the strength of the current in the solenoid was made so small 

 that the throws caused by reversing it could just be read, or so large 

 that they could hardly be measured on the scale, the ratio of the indi- 

 cations when the suspended coil was fixed and when it was free was 

 almost exactly the same ; but it was easy to magnify the efi"ect of the 

 electromotive force of the swinging coil by increasing the current in 



