42 



NATURE 



\_May 13, 1880 



his visit to the Cameroon Mountain?, and his ascent of the river 

 Binue. 



The Bolletino of the Italian Geographical Society for April 

 contains full details of the proposed Arctic Expedition under 

 Lieut. Bove, with a carefully compiled map of the south polar 

 regions so far as these have been hitherto explored. 



M. Desirii Charnay has left New York for IMexico for the 

 purpose of carrying out a thorough exploration of the ancient 

 remains that still exi4 in that country. It is expected that the 

 work of exploration will last for two or three years. 



ON ELECTRIC LIGHTING^ 



-£yYNAMO-ELECTRIC MA CniNES.— Since the date of 

 the author's former paper in April, 1S79, other observers 

 have published the results of experiments similar to those 

 described by him. It may be well to exhibit some of these 

 results reduced to the form he has adopted, viz., a curve, such 

 as that shown in Fig. 4, Procadings, 1879, Plate 29, and now 

 reproduced, with slight alterations, in Fig. i. Here any abscissa 

 represents a current passing through the dynamo-electric machine, 



and the corresponding ordinate represents the electromotive force 

 of the machine for a certain speed of revolution, when that 

 current is passing through it. It will be found (l) that with 

 varying speed the ordinate or electromotive force, corresponding 

 to any abscissa or current, is proportional to the speed ; (2) that 

 the electromotive force does not increase indefinitely with 

 increasing cunent, but that the curve approaches an asymptote ; 

 (3) that the earlier part of the curve is, roughly speaking, a 

 straight line, until the current attains a certain value, and that at 

 that point the electromotive force has reached about two-thirds 

 of its maximum value. When the current is such that the electro- 

 motive force ii not more than two-thirds of its maximum, a very 

 small change in the resistance w ith speed of engine constant, or 

 in the Sjieed of the engine with lesistance constant, causes a great 

 change in the current. For this reason : uch a current, w-hich is 

 the sanie for all speeds of revolution, since the curves for 

 different speeds differ only in the scale of ordinates, may be 

 called the " critical current " of the machine. The effect of a 

 change of speed is exhibited in Fig. i, where the \o\\ er line 

 represents a cm ve for a speed of 660 revolutions per minute, 



. , J c _ rr-y ■ . • electromotive force 



instead of 720. Ihe resis ance, varymg as 



current ' 



is given by the slope of the line OP, which must therefore be 

 constant ; and it will be seen that this line cuts the upper curve 

 nt a point corresponding to a current of 15 webers, and the lower 

 at a point corresponding to a current of 5 u ebers only. 



In Germany, Auerbach and Meyer [IVit'dcmann's AnnaUn, 

 November, 1879) have experimented fully on a Gramme ma- 

 chine at various speeds, and with various external resistances. 

 The resistance of the machine was 0*97 ohms. Their results are 

 summarised in a table at the end of their paper, which gives the 

 current passing, with resistances in circuit from i'75 to 200 

 Siemens units, and at speeds from 20 to 800 revolutions per 

 minute. In the accompanying diagram. Fig. 2, curve No. i, ex- 

 presses the relation between electromotive force and current, as 

 deduced from some of their i^servations, making allow ance, where 

 necessary, fir difference in speed. The curve, as actually con- 

 structed, is for a speed of 800 revolutions : at this speed it will 

 be seen that the maximum electromotive force is abiut 76 volts ; 

 the critical current, corresponding to a force of about 51 volts, 

 is 6'5 H ebers, with a total resistance of 7'8 ohms. Up to this 

 point there will be great instability, exactly as was the case in 



' Paper read at the Iiislilul'ion of Mechanical Engineers, by Dr. Jilin 

 Hopkinson, F.k.S. 



the Siemens macliine examined by the author, where the resist. | 

 ance was 4 ohms, and the speed 720 revolutions. 



The results of an elaborate series of experiments on certain 

 dynamo-electric machines have recently been presented to the 

 Royal Society by Dr. Siemens. One of the machines examined 

 was an ordinary medium-sized machine, substantially similar to 

 that tried by the author in 1879. It is described as having 24 

 divisions of the commutator ; 336 coils on the armature, with a 

 resistance of o'40i4 Siemens units ; and 512 coils on the mag- 

 nets, with a resistance of 03065 ; making a total resistance of 



soJ^UcircTTwtivc 



07079 Siemens units = o'6654 ohms. Curve No. 2 gives 

 the relation of electromotive force and current, reduced to a 

 speed of 700 revolutions per minute, the actual speeds r.mging 

 from 450 to 800 revolutions. The maximum electromotive force 

 appears to be probably 76 volts, and the critical current 15 

 webers, which is the same as in the author's first experiments on 

 a similar machine. 



In the summer of last year theauthor examined a Siemens machine 

 of the >mallest size. This machine is generally sold as an exciter 

 for their alternate current macliine. It has an internal resistance 

 of o"74 ohms, of which 0*395 '* '" '^^ armnture or helix. The 

 machine is marked to run at i, 130 revolutions per minute. The 

 following Table gives, for a speed of 1,000 revolutions, the total 

 resi-tance, current, electi'omotive force, and horse-power deve- 

 loped as current. The horse-power expended was not deter- 

 mined : — 



Exf-eriments on smaUest-sized Siemens Dynamo- Electric Machine 



Curve No. 3 gives as usual the relations of electromotive 

 force and current. From this curve it will be seen that 

 the critical current is 1 1 -2 webers, and the maximum electro- 

 motive force, at the speed of 1,000 revolutions, is about 42 volts. 

 The determinations for this machine were made in exactly the 

 same manner as in the experiments on the medium-sized machine, 

 using the galvannmeter, but omitting the ex|5eriment w ith the 

 calorimeter (compare Table I., p. 249, Proceed: 11 :;s, April, 1879). 



The time required to develop the current in a Gramme machine 

 has been examined by Ilerwig (Wiedemann, June, 1879). He 

 established the following facts for the machine he examined. A 

 reversed current, having an electromotive force of 0-9 Grove 

 cells, sufiiced to destroy the residual magnetisin of the electro- 

 magnets. If the re.-idual magnetism was as far as possible 

 reduced, it took a much longer time to get up the current than 

 when the machine was in its usual state. A longer time was 

 required to get up the current when the external resistance was 

 great, than when it w-as small. With ordinary resistance the 

 current required from f second to I second to attain its maximum. 



Br/q/i/itcss of ihe Electric Arc. — The measurement of the light 

 emitted by an eleciric arc presents certain peculiar difficulties. 

 The light itself is of a different colour from that of a standard 

 candle, in terms of which it is usual to e.xpress luminous intensi- 

 ties. The statement, without qualification, that a certain electric 

 lamp and machine give a li^ht of a specified nuiulier of candles, 

 is therefore wanting in definite meaninaf. A red light cannot 

 wi h propriety be said to be aiiy particular nmliiole of a green 

 liLdU ; nor can one light, which is a mixture of colours, be said 

 with strictness to be a multiple of another, unless the proportions 

 of the colours in the two eases are the ^ame Capt. Abney 

 {Procecdiii-i of the Royal Society, March, 1S7S) has given the 



