Fig. 544.--Queen weight voltameter for determining the strength of current by the weight of metal deposited in a given time. The two outside plates form the anode and are joined together and to one binding post, while the cathode is placed between them and connected to the other binding post. The cathode thus receives a deposit on both sides. An adjustable arm serves to lower the plates into the electrolyte. To calculate the strength of an unknown current which has passed through a weight voltameter, divide the gain in weight by the number of seconds the current flows through the instrument and by the weight deposited by one ampere in one second. That is, current strength in amperes = gain in weight ÷ (time in seconds × .0003286).
Ques. How much copper or zinc will one ampere deposit in one second?
Ans. .0003286 gramme of copper in a copper voltameter, or .0003386 gramme of zinc in a zinc voltameter.
Ques. What is the difference between an ampere and a coulomb?
Ans. An ampere is the unit rate of flow of the current, and a coulomb is the unit quantity of electricity, that is, the ampere is the rate of current flow that will deposit .0003286 grammes of copper in one second and a coulomb is the quantity of electricity that passes a given point in one second when the current strength is one ampere. In other words a coulomb is one ampere second.
Fig. 545.--Gas voltameter for determining the strength of current by the volume of gas evolved. To use, connect up as shown in the illustration. Adjust so that the zero position of the burette is about one-half inch below the level of the top of the U tube. Pour acidulated water into the mouth of the burette till the water in the U tube is about one-half inch from the top. With the electrodes inserted through the corks, carefully place each one in position by giving a slight twist to the right as the cork enters. The water level in the U tube and burette should now be the same, or further adjustment must be made to attain this result. The level in the burette does not necessarily have to correspond with the zero graduation, but must not be below it. Unclamp the burette and hold it nearly horizontal. The liquid will not run out if the corks be tight, so that this is the air leakage test. Attach the connectors and wires from the current source (which should have a pressure of 2 or more volts) placing a switch in the circuit. When the switch is closed, bubbles of gas will rise in the U tube from both electrodes, displacing the water and forcing it up the burette. Hydrogen will be liberated over the negative electrode, and oxygen over the positive electrode in the proportion of twice as much hydrogen as oxygen. To calculate the current strength, divide the volume of gas liberated by the time in seconds, and by the volume of gas liberated (in cubic centimeters) by one ampere in one second and by .1733; that is: amperes = volume of gas liberated ÷ (time in seconds × .1733).
EXAMPLE.--If an arc lamp require a current of 8 amperes, how much electricity does it consume per hour?
Since one coulomb = one ampere second, the quantity of electricity consumed per hour is
8 amperes × ( 60 × 60 ) = 28,800 coulombs.
Voltameter.--A voltameter is an electrolytic cell employed to measure an electric current by the amount of chemical decomposition the current causes in passing through the cell. There are two classes of voltameter: