404 



KNOWLEDGE 



[Nov. 



and external resistance?, but tliis is unavoidable in order 

 to satisfy the requirements of the lamp. We can only 

 retluce the external resistance by joining a number of 

 lamps in parallel circuit or multiple arc. This consists in 

 joining two or more lamps together in such a way that the 

 current coming from the battery or generator on reaching 



^ 



the junction, A, Fig. 1, of the lamps L, L' will divide 

 l>etween theiB, the two currents reuniting on emerging at 

 B. It will be apparent on a little rellection that looking 

 at a lamp as a conductor, two lamps put side by side or 

 in parallel circuit, double the conductivity of a circuit 

 having only one lauip, or, in other words, halve the resist- 

 ance. The e<|uations then become 

 40 



with 2 lamps 

 with 5 lamps 



3+1: 

 40 



; 2 2=1.1 1 per lamp 

 :4 4= -88 per lamp 



with 10 lamps ^ — = c 6= -06 per lamp 



In explanation it may be pointed out that although the 

 joint resistance of the external circuit in, say the first case, 

 is only 15 Ohms, each lamp oilers a resistance of 30 Ohms. 

 In the same way a water-pipe offers the same individual 

 resistance to a flow of water, when another similar pipe is 

 placed side by side with it to increase the delivery. The 

 pressure of water, or the E M F of the electric current, are 

 not affected by a reduction in the respective resistances, 

 and consequently the E M F will be the same through each 

 lamp, but as the current will have a choice of two similar 

 channels or circuit.^, the nmoxul of current will be ecjually 

 di«de<l l»etwecn them, so that we get only 11 Ampere 

 instead of 1 -2. This may be thought to be only a slight 

 difference, but w-e shall recognise its importance if we bear 

 in mind that we lose that part of the current which is 

 most effectivi- in illuminating the lamp. This loss of 

 luminosity continues as we add the lamp.s, until heat only 

 ia gencrat'ri. 



To secure sufficient current to furnish a strength of 1'2 

 Amper«-a per lamp, we must increase our battery power, 

 adding similar sets by the "quantity" arrangement, 

 ■o that 



40 

 with 2 lampK, 2 sets will give j -^ = 2 -4 2 = 1 - i per lamp. 



with lOlampg, sets will give -, =12 0=1-2 per lamp. 



The slight excess of current in the two first instances is 

 incn'SS'^i in the third instance to sufficient to enable us to 

 US'.- nine sets instead of t»n with which number we should 

 g«rt 1 -'J 1 f*r lamp. 



In each of the ctiei above cited the EM F was 

 tbe laise, but it must not be forgotten that when we 



join tlie lamps in series so that the whole current passes 

 through each lamp, then we must increase the E M F in 

 proportion. For instance, if wo wished to illuminate two 

 lamps in this way, we should require 40 cells, when we 



should get — i 1 o i 



Here, while we get the tlesired E M F, we also secure the 

 necessary amount of current ; because with a current 

 passing through a single circuit, the strength is the same 

 at each and every point in that circuit. 



An Ampere is for ordinary purposes too large to use as 

 a unit, and it has been found more practicable to use 

 instead a thousandth of an Ampere, under the designation 

 of a Milliampere (expressed frequently by the letters M A). 



The next unit for consideration is the Coulomb, which 

 is a current of one Ampere flowing for one second. There 

 is very little to be said on this head. It will be apparent 

 that whereas the Ampere is strength independent of time, 

 the Coulomb is strength enduring for a stipulated time, 

 and is therefore virtually a measure of quantity. We may 

 call it a measure of work-power. Again, as the force 

 necessary to raise one pound through one foot in one; 

 second is identical in amount with that necessary to 

 perform the same work in ten seconds, so also is one 

 Ampere lasting one second equal to a tenth of an Amjiiire 

 lasting ten seconds — that is, one Coulomb. 



TRANSITS OF VENUS. 

 By Rich ah d A. Proctor. 



(Cunliiivcd from jiuge 3ZG). 



"ITfE have now to consider tlie circumstances which 

 1 V render the varying position of the conjunction-lines 

 important in connection with transits of Venus. 



If Venus travelled in the same level as the earth, she 

 would necessarily be directly between the earth and sun 



Fig. 2. 



when in conjunction, as when the two bodies are at E„ V',, 

 or E^, V,,, or the like. Venus would then Vje visible, from 

 the hemisphere of the earth which was at the moment 

 turned sunwards, as a black spot centrally placed on thft 



