As soon as the acid has been prepared for the Voltaic cell fill a tumbler about three quarters full and then immerse the zinc and copper strips therein. As soon as the strips are in the acid, bubbles will commence to rise from the zinc. These bubbles are a gas called hydrogen and are evidence of a chemical action which takes place in a battery. The zinc is being dissolved by the acid and during the process, sets free hydrogen gas. It will probably be noticed that very few bubbles arise from the copper plate and that there seems to be little chemical action there. It will also be noticed that if the two wires connected to the strips are brought together the bubbles will arise from the zinc much faster than before. That is because, when the wires are connected together, a complete electrical circuit is formed: The zinc is really being oxidized or slowly burned. If zinc is burned in the open air or in a fire it will give out its energy in the form of heat but when it is burned in an acid solution in the presence of another metal it gives out its energy in the form of electricity. The zinc strip in a Voltaic battery is known as the *negative* pole or cathode, and the copper strip, as the positive pole or anode. When the electrical circuit is completed by touching the two wires connected to the poles together, the current is supposed to flow from the positive pole through the wires and back into the solution through the negative pole. If the two wires, instead of being connected together, are connected to an electrical instrument called a voltmeter the needle or pointer on the meter will swing over and point to about one volt. *A Voltmeter* is an instrument for measuring electrical pressure or *potential*. The pressure of an electric current is measured in *volts* just as the pressure of water may be measured in *pounds*. If the copper strip is lifted out of the solution and a carbon plate or rod also having a wire attached is substituted in its place it will be found that the voltage or potential has increased to one and one-half volts. Zinc and carbon are said to have a greater potential difference than zinc and copper and inasmuch as it is usually desirable for a battery to have the greatest potential difference possible, zinc and carbon are employed in the batteries of to-day instead of zinc and copper. If the wires are then disconnected from the voltmeter and connected to an electrical instrument called an *ammeter*, the needle or pointer will probably swing over until it indicates a current of perhaps ten amperes. An *ammeter* is an instrument for measuring the volume of an electric current. An *ampere* is a unit of current and is used to designate the rate of flow just as feet per second are used to denote rate of flow in the case of water in a pipe. If the meter is allowed to remain connected to the cell for a short time it will be noticed that the pointer will commence to slowly drop back towards zero.