SECTION LV. THE WATER VOLTAMETER. Apparatus required: Water Voltameter, two storage cells, tangent galvanometer (with a constant of 1 or 2), reversing key and resistance coils. When an electric current decomposes a liquid through which it is sent, the liquid is called an "electrolyte," and the process of decomposition is called "electrolysis." The laws of electrolysis state that the weight of an element set free from a compound by electrolysis, is proportional to the current, the time, the atomic weight, and inversely proportional to the valency of the element. One ampère flowing for one second liberates, e.g. 01118 grams of silver from any silver salt. The amount of an element liberated by a current in a given time may therefore be used to determine the magnitude of the current and hence to standardise a current-measuring instrument. In the present exercise a tangent galvanometer is to be standardised by means of a water voltameter. There are two ways of using a water voltameter, according as the hydrogen alone is measured, or both oxygen and hydrogen are collected together. In the latter case we gain by having a greater volume of gas to measure and being able therefore to reduce the time of the experiment, but there is some danger of error owing to the formation of ozone and its absorption in the water. It has however been shown by Kohlrausch that if currents of over one ampère are to be measured with an accuracy sufficient for commercial purposes, a voltameter in which both gases are collected together, will answer the purpose. For smaller currents the hydrogen only should be measured. The difficulty may also be overcome by using a solution of sodium hydrate (between 5 and 10%) with nickel electrodes. The volume of mixed gas produced by the passage of an ampère for a second is 1734 c.c. at normal temperature and pressure. The gas is collected in a graduated tube and its volume measured. To reduce the observed volume to the normal conditions, the temperature of the gas must be measured by a thermometer placed, if possible, inside the voltameter tube. The pressure is obtained by reading the barometer, by applying if necessary a correction for a difference of pressure inside and outside the voltameter tube, and by deducting the pressure of aqueous vapour in the tube. As pure water offers a high resistance to the electric current a little sulphuric acid is added to it. If the reductions are to be made to the highest possible accuracy, account must be taken of the fact that the pressure of saturated vapour of water is less over a solution of sulphuric acid than over pure water. But in the present exercise it is assumed that the acid added to the water is not sufficient to produce a sensible effect. One form of voltameter (Fig. 105) consists of two portions, a lower reservoir and a graduated tube which fits into it by means of a ground joint. The lower reservoir communicates with the atmosphere through an opening which can be closed by a glass stopper. During electrolysis this stopper must be removed. Pour water acidulated with about 15% of dilute sulphuric acid into the reservoir so as to render it about threequarters full. Insert the tube and stopper and tilt the voltameter so that the liquid runs into and completely fills the tube. Place the instrument in a tray and remove the stopper. Another form consists (Fig. 105 a) of a vertical tube graduated in c.c.s., from the bottom of which a narrower tube leads to the top of a tube similar to the first. By tilting the apparatus to the left through about 90°, the acidulated water can be made to fill both the graduated tube and the narrow tube. As gas is generated in the graduated tube the water expelled runs over from the top of the narrow tube into the second wide tube, and its level in the narrow tube remains constant. Fig. 105 a. Connect the galvanometer and voltameter in series with a reversing key, a resistance and two storage cells or other battery having an electromotive force of 3 or 4 volts and a low internal resistance. The object of the reversing key is to correct for errors of zero of the tangent galvanometer, by taking its deflections during half the time in one direction and half the time in the other. The key might therefore be inserted so as to affect the galvanometer only, leaving the current through the voltameter in the same direction throughout the experiment. Adjust the resistance so that when the circuit is closed the deflection of the galvanometer needle is about 45°. When this has been done, interrupt the current at the key, and wait till the bubbles of gas in the upper part of the column of liquid have ascended into the space above. Then read the position of the top of the column on the tube, measure to the nearest cm. its height above the liquid in the base, or in the second form of apparatus its depth below the top of overflow tube, and note the temperature of the gas. Read the zero of the galvanometer. Now make the circuit at the key, noting the time, and at the end of a minute read the galvanometer, repeating the reading every two minutes till the tube is about full of gas. One minute after taking the last reading, break the circuit, noting the time. After allowing the bubbles of gas in the column of liquid to ascend, read the position of the top of the column and measure as before the height of the liquid column, and the temperature of the gas. At a given instant make the circuit again, so that the current passes in the opposite direction through the galvanometer for an equal time. Read the galvanometer every two minutes as before, and the final position of the level of the liquid. Read the barometer. Calculate for each of the three observations of the column. of liquid, the volume of gas reduced to normal temperature and pressure, and subtract the first from the second and the second from the third. The differences are the volumes of gas produced in the observed times by the passage of the current. Take the mean of the tangents of the galvanometer deflections for each period. Then if d be the mean tangent, and K the constant of the galvanometer, the average current passing is Kd ampères. If v c.c. of gas were liberated in t seconds, then v V.Kd. t, where V is the volume of mixed gas liberated by 1 ampère in 1 sec., both being measured at the same temperature and pressure. = If v and V are the volumes reduced to a pressure of 76 cms. of mercury and a temperature of 0° C., we may substitute for V its numerical value 1734 and thus find Calculate K from the observation in this way, arranging the work as shewn below. The two sets of observations are reduced separately so as to afford a check on the calculations and a test of the consistency of the results. Remove the tangent galvanometer, and substitute for it one of the boxes used in the Magnetic Survey of the Laboratory (Section L.). Determine the time of oscillation of the needle. Determine also the time of oscillation of the needle when placed at some point of the Laboratory at which the earth's horizontal magnetic force H is known, and from the observations calculate the value of H at the place where the galvanometer stood. From the value of H and the radius and number of turns of the coil calculate the constant of the galvanometer (see Section LI.), and compare the calculated value with the result of the experiment. |