On the Constant of an Electro-dynamometer. Ill 



the sun, the temperature of the surface can never be raised 

 above the freezing-point while the ice remains ; and, again, 

 that such an ice-covering tends to its own preservation, because 

 it chills the air and increases the snowfall. In short, I have 

 all along maintained this to have been one of the chief causes 

 which led to the country being so deeply covered with ice. 

 In fact, had it not been for some such conservative power in 

 the ice, a glacial.epoch resulting from the causes which I have 

 been advocating would not have been possible. This conser- 

 vative tendency certainly renders it more difficult for the 

 physical agencies to get rid of the ice during interglacial 

 periods; but we evidently have no grounds for assuming that 

 it w T ill defy their melting-powers. 



I shall next consider Geological and Palaeontological Facts 

 in relation to Mr. Wallace's modification, and also his theory 

 as to the cause of Mild Arctic Climates. 

 ,[To be continued.] 



XIII. On a Method of determining experimentally the Constant 

 of an Electro-dynamometer. By A. P. Chattock*. 



THE practical importance attaching to the accurate mea- 

 surement of electric currents is daily increasing with 

 the extended use of electricity. For this reason I venture to 

 hope that the following account of some experiments on the 

 calibration of an electro-dynamometer will prove interesting. 



The twisting moment with which the fixed coil of an electro- 

 dynamometer acts on the suspended coil is proportional to the 

 product of the strengths of current flowing through the two 

 coils respectively, and to a factor which depends on their geo- 

 metrical relations. 



If a current be sent through the fixed coil and the suspended 

 coil be allowed to rotate, an B.M.F. is set up in the latter 

 which is at any instant proportional to the speed of rotation, 

 to the strength of the current in the fixed coil, and to a factor 

 depending on the geometrical relations of the two coils. 



By allowing the suspended coil to rotate about its axis of 

 suspension, and by making the geometrical factor during 

 rotation the same as afterwards, when the coil is suspended in 

 its place, I have determined the constant of the instrument 

 upon the table. 



It is the method I employed in doing this which I have the 

 honour of bringing before you to-day. The diagram shows the 

 arrangement of the apparatus. 



* Communicated by the Physical Society, having been read Novem- 

 ber 24, 1883. 



