172 Mr. G. J. Burch. On the Time- Relations [Nov. 19, 



the wires, and an error of hundreds per cent, in the case of some of 

 them. 



Using the formula (3) which we have arrived at for determining 

 the emissivity of platinum wires of different diameters at 300 C., it 

 follows that to maintain a platinum wire 07.5 mil in diameter at 

 300 C. would require a current density of 331,000 amperes per 

 square inch, and, if the emissivity of a copper wire of the same 

 diameter and at the same temperature may be taken as being the 

 same, it follows that to maintain a copper wire 0'75 mil in diameter 

 at 300 C. would require a current density of 790,000 amperes per 

 square inch. 



II. " On the Time-Relations of the Excursions of the Capillary 

 Electrometer, with a Description of the Method of using it 

 for the Investigation of Electrical Changes of Short Dura- 

 tion." By GEORGE J. BURGH, B.A. Oxon. Communicated 

 by Professor BARTHOLOMEW PRICE, F.R.S. Received Sep- 

 tember 3, 1891. 



(Abstract.) 



This papor is in continuation of the author's preliminary note " On 

 a Method of determining the Value of Rapid Variations of a Differ- 

 ence of Potential by means of the Capillary Electrometer," and 

 describes a further simplification of the method then brought forward, 

 consequent on a change in the mode of producing the photographic 

 record of an excursion. 



The rapidity of the movement of the meniscus was found to be 

 affected by (1) the degree of concentration of the acid, (2) the length 

 of the capillary beyond the end of the mercury column, (3) the shape 

 of the tube where it tapers to form the capillary, (4) the shape of the 

 orifice. These things might be taken as indicating the action of both 

 mechanical friction and electrical resistance in determining the rate 

 of movement. As was announced in the preliminary note, under 

 ordinary circumstances the instrument is perfectly dead-beat; the 

 meniscus commences to move the instant v difference of potential is 

 communicated to the instrument, and stops directly it is withdrawn. 

 The conditions under which overshooting may occur, and the possible 

 extent of it, are discussed. It was found that, in general, the time- 

 relations of the movement might be expressed by the equation 



y = ae 



in which y is the distance of any point upon the curve from its 

 asymptote. The tabular logarithms of a series of ordinates corre 



