ELECTRICAL INVENTIONS 231 



held in an absorbent substance as water is held by a sponge. 

 A cylindrical copper or sheet zinc cup is nearly filled with a 

 moist mixture of ammonium chloride, manganese dioxide, and 

 charcoal, each powdered. A rod of carbon is placed in the center 

 of the cup, whose open end is then sealed with asphalt or a 

 similar substance, through which the rod protrudes. One of the 

 binding-posts foi the wires is attached to this projecting rod, 

 the other to the copper or sheet zinc cup. The ammonium 

 chloride reacts with the copper giving ammonia and copper 

 chloride, which later ionizes. The ammonia is oxidized by the 

 manganese dioxide which becomes the simple oxide. The charcoal 

 serves to hold the moisture and to absorb excess of gases formed. 



The so-called storage battery commonly used in automobiles 

 to furnish current for the starter and for the spark plugs is not 

 a battery in the same sense as those described. It does not 

 produce electric energy, but merely stores it. 



Early in the nineteenth century it was accidentally discovered 

 that when a current of electricity is sent from a strong battery 

 into a weak one, the latter becomes charged and will, when used, 

 give off a relatively strong current. It was not, however, until 

 in 1859, when Plante discovered the peculiar adaptability of 

 lead for use in the storage battery or accumulator, that such 

 batteries became really serviceable. The principle of operation 

 is simple. When a current is sent into a storage battery, its 

 energy is there used to accomplish certain chemical changes. 

 Then, when this charged battery is used, these chemical changes 

 reverse, and the battery gives off the electric current that was 

 used in their production. This current comes off in a reverse 

 direction from that of the charging current. 



The most commonly used storage battery (Fig. 98) consists 

 of two sets of lead plates, those of one set closely alternating with 

 those of the other, and all immersed in dilute sulphuric acid 

 (15-30 per cent in distilled water). When the battery is being 

 charged, one set of plates is connected with the positive pole 

 (anode), and the other with the negative pole (cathode), of a 



