NO. 2 HISTORY OF ELECTRIC LIGHT — SCHROEUER 5 



electricity in reasonable quantities was made available, so the Volt, 

 the unit of electrical pressure was named in his honor. It was later 

 shown that the chemical affinity of one of the metals in the liquid 

 was converted into electric energy. The chemical action of Volta's 

 battery is that the salt water attacks the zinc when the circuit is 

 closed forming zinc chloride, caustic soda and hydrogen gas. The 

 chemical equation is : 



Zn + 2NaCl + 2U0O = ZnClo + 2NaOH + Ho 



improvement of volta's battery 



It was early suggested that sheets of silver and zinc be soldered 

 together back to back and that a trough be divided into cells by these 

 bimetal sheets being put into grooves cut in the sides and bottom of the 

 trough. This is the reason why one unit of a battery is called a " cell." 

 It was soon found that a more powerful cell could be made if copper, 

 zinc and dilute sulphuric acid were used. The zinc is dissolved by 

 the acid forming zinc sulphate and hydrogen gas, thus: 



Zn + H2SO4 = ZnS04 + Ho 



The hydrogen gas appears as bubbles on the copper and reduces the 

 open circuit voltage (about 0.8 volt per cell) as current is taken from 

 the battery. This is called " polarization." Owing to minute im- 

 purities in the zinc, it is attacked by the acid even when no current is 

 taken from the battery, the impurities forming with the zinc a short 

 circuited local cell. This is called " local action," and this difficulty 

 was at first overcome by removing the zinc from the acid when the 

 battery was not in use. 



Davy's discoveries 



Sir Humphry Davy was a well-known English chemist, and with 

 the aid of powerful batteries constructed for the Royal Institution in 

 London, he made numerous experiments on the chemical efifects of 

 electricity. He decomposed a number of substances and discovered 

 the elements boron, potassium and sodium. He heated strips of 

 various metals to incandescence by passing current through them, 

 and showed that platinum would stay incandescent for some time 

 without oxidizing. This was about 1802. 



In the early frictional machines, the presence of electricity was 

 shown by the fact that sparks could be obtained. Similarly the break- 

 ing of the circuit of a battery would give a spark. Davy, about 1809, 

 demonstrated that this spark could be maintained for a long time with 



