CHAP, in.] ELECTRICITY AND MAGNETISM. 179 



The galvanometer shows, however, the strength of the 

 current at any moment, and its variations in strength 

 from one moment to another, by the position of the 

 needle. In the Voltameter, a varying current may 

 liberate the bubbles of gas or the atoms of copper rapidly 

 at one moment, and slowly the next, but all the varying 

 quantities will be simply added together in the total 

 yield. In fact, the voltameter gives us the " time 

 integral " of the current. It tells us what quantity of 

 electricity has flowed through it during the experiment, 

 rather than how strong the current was at any one 

 moment. 



218. Chemical Test for Weak Currents. A 

 very feeble current suffices to produce a perceptible 

 amount of change in certain chemical substances. If 

 a few crystals of the white salt iodide of potassium are 

 dissolved in water, and then a little starch paste is added, 

 a very sensitive electrolyte is obtained, which turns to 

 an indigo blue colour at the anode when a very weak 

 current passes through it. The decomposition of the 

 salt liberates iodine at the anode, which, acting on the 

 starch, forms a coloured compound. White blotting- 

 paper, dipped into the prepared liquid, and then laid on 

 the kathode and touched by the anode, affords a con- 

 venient way of examining the discoloration due to a 

 current. A solution of Ferrocyanide of Potassium affords 

 similarly on electrolysis the well-known tint of Prussian 

 Blue. Bain proposed to utilise this in a Chemical 

 Writing Telegraph, the short and long currents trans- 

 mitted along the line from a battery being thus recorded 

 in blue marks on a strip of prepared paper, drawn along 

 by clockwork under the terminal of the positive wire. 

 Faraday showed that chemical discoloration of paper 

 moistened with starch and iodide of potassium was pro- 

 duced by the passage of all different kinds of electricity 

 frictional, voltaic, thermo-electric, and magneto -electric, 

 even by that evolved by the Torpedo and the 



