NATURE 



[March 28, 1895 



of Aldebaran up lo l' to left of Japiter, moving slowly west- 

 wards a< before." 



Mr. G. W. Lamplugh observed the display at Ramsay, Isle 

 of Man, and t) him the chief feature was " the predominance 

 of a well-defined luminous bar extending acro.'S the heavens 

 directly overhead from one horizon to the other — from magnetic 

 east to west. 



"The stars shone through this band of pale light with 

 scarcely diminished brightness, but it was occasionally flecked 

 by thin clouds. When I first saw it, at 9 20 p.m., there seemed 

 to be traces of oblique striation crossing it, and Dr. Tellet, 

 who called my attention to the display, slates that ten minutes 

 earlier, when the bar was at its brightest, these strialions gave 

 the effect of slightly twisted folds. Another informant remarked 

 that the bar was formed shortly before 9 p.m. by the union over- 

 head of rays which shot upwards for the east and west. 



" The light waned rather rapidly with ?light pulsations, and 

 we thought that we could detect a slow southerly drift in the 

 band before it vanished. The western portion died out before 

 the eastern, which remained quite detinite until 9"45 p.m. 

 Meanwhile there had been a faintly diffused illumination of 

 the northern heavens, with occasional suggestions of radiant 

 streamers, but the whole quite subordinate in brightness to the 

 band overhead. 



" There was a westerly breeze and a nearly clear sky, with a 

 low cloud-bink in the north-west, at the commencement of the 

 display, but before 9'45 p.m. the wind had backed southward, 

 and shortly afterwards the sky became suddenly overcast, though 

 not before the aurora had faded to a scarcely perceptible glow. 

 Half an hour later a slight shower fell." 



The Rev. S. Barber says that the aurora was visible at Wesl- 

 newtOD, Aspatria, in great brilliancy, as a band passing nearly 

 over the zenith from the west to the east horizon at about 

 10 p.m. .\n arch of light was seen in the north from about 

 8.30, and some observers saw a shorter band almost north and 

 south preceding the great band west and east. 



Mr. J. Cuthbertson saw the display at Kilmarnock, N.B., as 

 early as 7.45, "as a broad arch of light crossing the heavens 

 from east to west. .Vbaut 7.50 it was a very luminous pencil 

 of light in the upper part of the heavens, through which 

 stars of the second and third magnitude were distinctly visible, 

 As it condensed it grew brighter. .About ten minutes after I 

 first observed it, it began to fade, and was invisible before 8 

 o'clock, leaving a temporary brightness in the western sky." 



The aurora appears to have presented features very similar to 

 those of the aurora of November 23, 1894, observations of 

 which were discussed by Prof. A. S. Hetschel in Natitre of 

 January 10. 



THE U.S. UNITS OF ELECTRICAL MEASURE. 

 "D Y a law approved in the Senate of the United States, last 

 July> '' *"5 enacted that the legal units of electrical 

 measure in the United States should be as follows : — 



(1) The unit of resistance shall be what is known as the 

 international ohm, which is substantially equal to one thousand 

 million units of resistance of the centimelre-gramsecond system 

 of clectrcmagnelic units, and is represented by the resistance 

 offered to an unvarying electric current by a column ai mercury 

 at the temperature of melting ice fourteen and four thousand 

 five hundred and twenlyone ten-thousandths grams in mass, 

 of a constant cross^-ectional area, and of the length of one 

 hundred and six and three-tenths centimetres. 



(2) The unit of current shall be what is known as the inter- 

 national ampere, which is one-tenth of the unit of current of 

 the ccntimetre-Kramsccond .'•ystem of electromagnetic units, 

 and is the practical equivalent of the unvarying current, which, 

 when passed through a solution nf nitrate of silver in water in 

 accordance with standard specifications, deposits silver at the 

 rate of one thousand one hundred and eighteen millionths of a 

 gram per second. 



(3) The unit of electromotive force shall be what is known 

 as me international volt, which is the electromotive fjrce that, 

 steadily applied to a conductor whose resistance is one inter- 

 national ohm, will produce a current of an international ampere, 

 and is practicilly equivalent to one thousand fourteen hundred 

 and thirty- fourths ol the electromotive force between the poles 

 or electrodes of the voltaic cell known as Clark's cell, at a 

 temperature of fifteen degrees Centigrade, and prepared in the 

 manner described in the standard specifications. 



NO. 1326, VOL, 51] 



(4) The unit of quantity shall be what is known as the inter- 

 national coulomb, which is the quantity of electricity transferred 

 by a current of one inlcrnational ampere in one second. 



(5) The unit of capacity shall be what is known as the inter- 

 national farad, which is the capacity of a condenser charged 

 to a potential of one international volt by one international 

 coulomb of electricity. 



(6) The unit of work shall be the Joule, which is equal to ten 

 million units of work in the centimetre-gram-second system, 

 and which is practically equivalent to the energy expended in 

 one second by an international ampere in an international ohm. 



(7) The unit of power shall be the watt, which is equal to 

 ten million units of power in the centimetregram-second system, 

 and which is practically equivalent to the work done at the rate 

 of one Joule per second. 



(8) The unit of induction shall be the Henry, which is the 

 induction in a circuit when ihe eleclromo'.ive force induced in 

 this circuit is one international volt while the inducing current 

 varies at the rate of one ampere per seconi. 



The National Academy of Sciences was instructed to prescribe 

 and publish the specifications necessary for the practical appli- 

 cation of the definitions of the ampere and volt given in the fore- 

 going,iand, to meet this requirement of Congress, a special com- 

 mittee was appointed to consider the subject. The committee, 

 selected from members of the .\cademy, w.as as follows : — Prof. 

 H. .\. Rowland, chairman. General II. L. Abbot, Prof. G. F. 

 Barker, Prof. C. S. Hastings, Prof. A. A. Michelson, Prof. J. 

 Trowbridge, Dr. Carl Barus. 



The report of this committee was submitted to the Academy 

 at a special meeting held last month, and was then accepted and 

 unanimously adopted. We extract the following details from the 

 report, a copy of which has just reached us. 



The Am pen. 



In employing the silver voltameter to measure currents of 

 about one ampere, the following arrangements shall be 

 adopted : - 



The kathode on which Ihe silver is to be deposited shall take 

 the form of a platinum bowl not less than 10 centimetres in 

 diameter, and from 4 to 5 cenliniLlrcs in depth. 



The anode shall be a disc or plate of pure silver some 30 

 square centimetres in area and 2 or 3 millimetres in thickness. 



This shall be supported horizontally in the liquid near the top 

 of the solution by a silver rod riveted through its centre. To 

 prevent the disintegrated silver which is formed on the anode 

 Irom falling upon the kathode, the anode shall be wrapped 

 around with pure filter paper, secured at the back by suitable 

 folding. 



The liquid shall consist of a neutral solution of pure silver 

 nitrate, containing about 15 parts by weight of the nitrate to 

 85 parts of water. 



The resistance of the voltameter changes somewhat as the 

 current passes. To prevent these changes having too great an 

 effect on the current, some resistance besides that of the volta- 

 meter should be inserted .in the circuit. The total metallic 

 resistance of the circuit should not he less than 10 ohms. 



Method of iiutkiiig a Memurcmcnt. The platinum bowl is to 

 be washed consecutively with nitric acid, distilled water, and 

 absolute alcohol ; it is then^to be dried at 160 C, and left to 

 cool in a desiccator. When thoroughly cool it is to be weighed 

 carefully. 



It.is to be nearly filled with the solution and connected to the 

 rest of the circuit l)y being placed on a clean insulated copper 

 support to which a binding screw is attached. 



The anode is then to be immerse<l in the solution so as to be 

 well covered by it and supported in that position ; the connec- 

 tions to the rest of the circuit are then to be made. 



Contact is to be made at the key, noting the lime. The 

 current is to be allowed to pass for not less than half an hour, 

 and the lime of breaking contact observed. 



The solution is now to be removed from the bowl, and the 

 deposit washed with distilled water, and left to soak fir at least 

 six hours. It is then to be rinsed successively wiih distilled 

 water and absolute alcohol, and dried in a hot air bath atatem- 

 pcral«re of about 1 60° C. Alter cooling in a desiccator it is to be 

 weighed again. The gain in mass gives the silver deposited. 



To find the time aver.ige of the current in amperes, this mass, 

 expressed in grams, must be divided by the number of seconds 

 during which Ihe current has passed, and by 0001 1 iS. 



In <lelermining the constant of an instrument by this method. 



