164 



KNOWLEDGE. 



May, 1913. 



necessarily acts upon the metals and so limits the 

 life of the battery. Yet acid is necessary because 

 without it water is not a good enough conductor 

 for the current. 



Whether Volta was strictly accurate in looking 

 upon chemical action as the result rather than the 

 cause of the electricity in a voltaic cell may be an 

 open question, but certainly his explanation may be 

 as good as the other, and it is noteworthy that it is 

 sometimes instinctively adopted even by those who 

 believe in the chemical theory ; for it is usual to 

 speak of the corrosion that takes place when zinc 

 and copper are rivetted together as being due to 

 electric action. 



But what naturally will occur to anyone hearing 

 of the suggestion that zinc and copper give 

 electricity by mere contact and that the acid 

 solution is a mere conductor which is only 

 incidentally destructive of the plates, is the question, 

 why should not the metals be used without the acid 

 and a conducting way for the current be found by 

 uniting the pairs of metals direct ? In this way it 

 would appear at first sight as though we could get a 

 series of cells without any liquid conductor, and one, 

 therefore, practically permanent. 



A moment's consideration will, however, show 

 that this is not practicable. If when zinc is joined 

 to copper a current flows from the zinc to the 

 copper, it is evident that by joining another pair a 

 current will be established in the opposite direction. 

 The following diagram, in which the arrows show 

 the direction of flow of the current from contact 

 electricity, will make this clear. : — 



ZCZCZCZC, and so on. 

 — > — > — >■ — > 



The contacts obviously involve currents in the 

 opposite direction which will neutralise the others. 



Nor is any better result obtained if a wire of some 

 other metal, say iron, is used to connect the pairs. 

 In this case the effect is shown by the arrows in the 

 following diagram (the numbers representing current 

 intensity between the various metals) : — 

 11 ii ii 



ZCIZC1ZCIZC. 



— > — >- — >- — >• 



2 2 2 2 



In fact the following law has been abundantly 

 established : — When any number of metals are 

 placed in contact in a series that returns to the metal 

 it started with, there is always an equal development 

 of contact electricity in each direction and conse- 

 quently no flow of current either way. 



However, it is said that Nature never locks a door 

 without also providing a key. The key or clue to 

 this problem consists in finding a uniting medium 

 for the pairs of metals which is non-metallic, and at 

 the same time free from chemical action, while 

 sufficiently conductive to allow electricity to pass. 

 Such a conductor is found in water. Its resistance 

 without acid is high, but still it has some conducting 

 power, while its contact electricity with the metals 



does not appear to be sufficiently appreciable to 

 neutralise the flow. Reverting to our former 

 diagram the series will now be 



Z C W Z C W Z C, and so on. 



— > — > — > 



It is evident that in an extended series, there being 

 a minute difference of electric potential between each 

 pair, there will be a considerable potential difference 

 between the extremities of the series. 



The experiment ma}' be made with a hundred 

 test tubes in which copper and zinc soldered strips 

 are placed in order. (See Figure 150). 



If these are filled with pure distilled water 

 practically no chemical action will occur, but the 

 ends of the series will be in so high a degree charged 

 with opposite electricities that the gold leaves of an 

 electroscope will diverge widely when connected with 

 either terminal. One end of the series will show a 

 positive charge, the other a negative charge. 



So far from this being due to any chemical action, 

 it will be actually found that the potential difference 

 is greater when distilled water is used than when 

 acidulated water is placed in the tubes. 



With a water battery of this form made of some 

 thousands of tubes, Leyden jars may be charged and 

 sparks of considerable intensity may be obtained. 



The difficulties attending the construction of such 

 a series and of insulating each tube perfectly are, 

 however, very great, and when an extended series 

 numbering thousands is required it is more conveni- 

 ent to resort to what is called the dry pile, though it 

 is not strictly speaking quite dry ; if it were it would 

 not work. 



The dry pile substitutes a connecting medium of 

 ordinary paper for the distilled water, and so slight 

 is the amount of moisture required for conduction 

 that the small quantity of water always present in 

 the pores of ordinary paper will suffice. For the 

 metal plates it is unnecessary to cut out metal 

 strips. Paper with a metal surface can be obtained 

 read)' made, either bronzed or tinned. Cut into 

 squares or punch into circles a series of bronzed 

 and tinned papers, keeping them in uniform order 

 so as to have the arrangement — tin, paper, paper, 

 bronze, tin, paper, paper, bronze, and so on, and the 

 dry pile is made. The heap is firmly pressed together 

 between two pieces of well-varnished glass for 

 insulation, and close contact is maintained either 

 by binding the whole round with tape or by gluing 

 a strong band of brown paper round it. The 

 terminals are strips of thin metal to which wires 

 have been soldered. A series made up of twenty 

 or thirty sections and well insulated will show 

 undiminished electric activity for years. The 

 energy will not be sufficient to give a spark, but 

 the attractive power of the terminals will keep a 

 small brass ball (suspended by a hair or fine silk) 

 oscillating between two bell gongs without cessation 

 year after year with no signs of exhaustion or 

 enfeeblement. 



A valuable practical application of the dry pile is 

 found in an instrument for detecting minute 



