♦ KNOWLEDGE ♦ 



[Oct. -20, 1882. 



THE AMATEUR ELECTRIC lAX. 



KI.KCTIUCAl. MK.V.SIREMKXT.— IV. 



HAVING explained Uie Volt and the OIuii, we now 

 come to a (juestion equally important, but perhaps a 

 littlf more intrioato, viz., the gtmii/t/i of t/ir mrni,/. The 

 nnit strengtli is called an Amptre, and is that amount of 

 lurrent nsulting from an electromotive force of one Volt 

 passing through a total n'sistance of one Olim. By the 

 total resistance is n\eant the resistance of the battery as 

 'veil a.'i the wire ; and whatever instruments may be in the 

 itvuit, so that the ordinary Daniell cell, while it has an 

 : -M F (electromotive force) of about one Volt, will notpro- 

 uice a current of one Ampere, liecause the resistance of the 

 cell is more than one Ohm ; it is, in fact, about ten Ohms. 

 fiuch a cell, having its poles connected by a short thick 

 strap of copper, would give a current equal to one tenth of 

 Ml Ampere. This is best seen by reference to the world- 

 ■fametl Ohm's Law. By some mysterious means this law, 

 doubtod and denied for half-a-century, now fully borne out 

 by experiment and accepted by every electrician, is by the 

 tyro or the student regarded with feelings somewhat akin 

 to awe, whereas it is in reality one of the simplest, most 

 beantiful, and most universally applicable in the whole 

 range of scientific inquiry. This great law is expressed by 



the equation ij =C!. 



'•r, in other words, the electromotive force (in Volts) 

 divided by the resistance (in Ohms) gives us the strength 

 of the current (in Amperes). The resistance embraces, as we 

 said, all re.sistances in the circuit. 



We will endeavour to simplify matters Ity taking a few 

 examples. The resistance of a Daniell cell being 10 Ohms, 

 lit cells will offer 100 Ohms, .".0 cells .'>00 Ohms, and so on. 

 r.y joining the two ends of our battery together by means 

 of a short piece of thick wire, so as to oiler practically no 

 external resistance, we shall get 



ith one eel! _- = — : = 1 



with 10 ce' 



with 50 



R~10 

 E_ K)^ 

 I{~100^ 



and so on, showing that we get in each case -1 of an 

 Anipi-re, and that to all intents and purposes one cell is as 

 -ooti OS 100 cells. If, however, we insert resistance in the 

 xK-mal circuit, in the form of a telegrai>h win- and in- 

 trunients the rehult will be very diflerent. Suppose wo 

 Krtvp a linf offering 100 Ohms! Then, letting R=tho 

 re«i8tanc« of the battery, and 11' = the resistance of the 

 line, we gr-t 



with one cell ^%,= ^}j^^= 00^ 



wiUi 10 cells ^ = IP = 050 



U + R' 100+100 "^" 



with 20 ceiU 



with .'lO cells 



■or,G 



_E__ 20 



R + R'~-'O0+100' 

 _E .^.0 _ . 



R + R'~ 500+100" '^^^ 

 111 the first two instances the strength of the resulting 

 urr<nt increases considerably with the number of cells 

 Tnployerl, but in the two latt<r cases this proportion is not 

 Maintained, owing to the relative reduction in the value or 

 Miiportance of the line resistance. Taking a longer line, 

 ay I>ondon to Newcastle, offering 5,000 Ohms, including 



300 Ohms for the resistance of tin 

 get 



receiving apparatus, we 



T=-00109 



;= 01 92:4 



- -OOSSt 



5000+lOUU- ■'^'^'^'^^ 

 The increase in the strength of the current is now seen 

 to be advancing nearly in proportion to the number of 

 cells. The ratio would be more striking if we wore to take 

 the cells actually in use for long telegraph linos, ort'erin;,' 

 only 2 Ohms resistance per cell. Thus we get 



with 10 cells !!!_ 



5000 + 20 



with 100 cells -— 



5000 + 200" 



These simjilo calcnlations demonstrate, then, that so long; 

 as the external resistance exceeds that of the battery, it is 

 economically adxantageous to incrt-ase thi; number of cells 

 joined in succession or series. 



We come next to consider the case in which we have a 

 low external resistance, but our electrical effect being too 

 small, we want to increase the current. How shall we do it ? 

 Our tii-st examples demonstrated that we could not advan- 

 tageously increase the cells in scries. It is clear that the 

 only way by which we can accomplish this is to reduce the 

 denominatiir of ,',-., or, in other words, reduce the resistance 

 of the battery. This can be done by increasing the size of 

 the plates, and so bringing a larger amount of surface into 

 action. That this should be so, will be apparent if we cavi 

 bring ourselves to look upon the cylinder of water between 

 the plates as a wire. Then if wc double the area of the 

 plates we double the sectional area of the cylinder of water 

 between them, and " the resistance of a conductor varies in 

 the inverse proportion to its sectional area." Supposing 

 the 10 Ohms cell to have plates oll'ering square inches of 

 surface to the liquid, a cell with 12 squai-e inches will offer 

 only 5 Ohms. Here, instead of getting a current 



^=1-1 



T> 11-1 ' 



we should get — = . = -2 



that is to say, twice as strong. It woulil, however, be very 

 inconvenii^nt in many ways, were it necessary to resort to 

 such a process. The same rc>sult can be obtained much 

 more easily by the process generally known as joining 

 up for "quantity." It consists in connecting the zinc or 

 positive plate of one cell to the zinc of another, and like- 

 wise joining the two negative plates together. Fig. 2 will 

 illustrates this. B' and B" arc two batteries, each con- 

 sisting of 5 cells, the short thick strokes representing the 

 zinc plates, and the long thin ones the copper plates. 



The terminal plates C C on thf^ right side are connected 

 by a piece of wire; ZZ on the left side being similarly 

 connected. When batteries are joined up in this way they 

 should consist of equal numbers of equally ellicient cells, 

 otherwise there will be a tendi-ncy of the current from th 

 stronger battery to run through the weaker, and soon 

 render it useless. ' Wires are fastened at M and F and 

 joined to whatever apparatus the current is required for. 

 Suppo.se we wish to send it through a Galvanometer G, 

 made l>y winding a fi-w turns of thick win; round a mag- 

 netic needle. The external resistance in this case will bo j 

 practically nil. Conse(juentIy tlie deflection of the needle 



