ELECTRICAL MEASUREMENTS. 267 



increases, but, if you multiply the force by the cube of 

 the distance, then the product approaches more and more 

 nearly to a constant value as the distance becomes greater, 

 and if the current is one of unit strength, and the two 

 areas are equal to unity, the constant value to which this 

 product approaches as the distance increases is unity. But, 

 as you will see, the considerations involved are rather 

 complex, and the further discussion of this subject would 

 take much more time than we can spare. 



Then another action produced by currents which might 

 be adopted, and often is adopted, for measuring their 

 strength, is the amount of chemical change which they can 

 produce in a given time ; or, to express it more concisely, 

 the strength of a current can be measured by the rate at 

 which it can produce chemical change in a body through 

 which it is passed by the rate, for instance, at which it 

 can decompose water or any other chemical compound. 

 Adopting this system we may take as the unit current one 

 of such a strength that it will decompose unit mass of a 

 standard substance say water in unit of time say a 

 second. We may define unit current as a current which 

 will decompose a grain of water in a second, taking a 

 grain as the unit of mass, water as the standard substance, 

 and a second as the unit of time. But if we are to adopt 

 this definition, the unit current obtained in this way would 

 not agree with the unit current derived from the electro- 

 magnetic action. An electro-magnetic unit current (re- 

 ferred to centimetres, grammes, and seconds), if used to 

 decompose water, will liberate '0001052 gramme of 

 hydrogen per second, or it would decompose nine times 

 that quantity of water. This gives us the ratio between 

 the electro-magnetic unit current and this electrolytic 

 unit. The apparatus employed for carrying out this kind 

 of measurement is of various kinds. They are called Volta- 

 meters ; many forms are familiar to all who have dealt at 

 all with electrical processes, and I will, therefore, merely 

 speak of one form which may be new to most if not all of 

 you a very beautiful instrument, constructed by Professor 

 Lenz, of St. Petersburg, which Baron von Wrangel has 

 kindly explained to me. It is an apparatus in which a 

 basic mercurous nitrate is decomposed. The current- is 

 passed through two little glass vessels containing mercury, 



