The next step of importance is to _measure_ the current, that is, to determine its strength or intensity, as well as the flow or quantity. DIFFERENT WAYS OF MEASURING A CURRENT.--There are several ways to measure the properties of a current, which may be defined as follows: 1. THE SULPHURIC ACID VOLTAMETER.--By means of an electrolytic action, whereby the current decomposes an acidulated solution--that is, water which has in it a small amount of sulphuric acid--and then measuring the gas generated by the current. 2. THE COPPER VOLTAMETER.--By electro-chemical means, in which the current passes through plates immersed in a solution of copper sulphate. 3. THE GALVANOSCOPE.--By having a coil of insulated wire, with a magnet suspended so as to turn freely within the coil, forming what is called a galvanoscope. 4. ELECTRO-MAGNETIC METHOD.--By using a pair of magnets and sending a current through the coils, and then measuring the pull on the armature. 5. THE POWER OR SPEED METHOD.--By using an electric fan, and noting the revolutions produced by the current. 6. THE CALORIMETER.--By using a coil of bare wire, immersed in paraffine oil, and then measuring the temperature by means of a thermometer. [Illustration: _Fig. 41._ ACID VOLTAMETER] [Illustration: _Fig. 42._ COPPER VOLTAMETER] 7. THE LIGHT METHOD.--Lastly, by means of an electric light, which shows, by its brightness, a greater or less current. THE PREFERRED METHODS.--It has been found that the first and second methods are the only ones which will accurately register current strength, and these methods have this advantage--that the chemical effect produced is not dependent upon the size or shape of the apparatus or the plates used. HOW TO MAKE A SULPHURIC ACID VOLTAMETER.--In Fig. 41 is shown a simple form of sulphuric acid voltameter, to illustrate the first method. A is a jar, tightly closed by a cover (B). Within is a pair of platinum plates (C, C), each having a wire (D) through the cover. The cover has a vertical glass tube (E) through it, which extends down to the bottom of the jar, the electrolyte therein being a weak solution of sulphuric acid. When a current passes through the wires (D), the solution is partially decomposed--that is, converted into gas, which passes up into the vacant space (F) above the liquid, and, as it cannot escape, it presses the liquid downwardly, and causes the latter to flow upwardly into the tube (E). It is then an easy matter, after the current is on for a certain time, to determine its strength by the height of the liquid in the tube.