almost entirely prevent the annoyance due to acid spray during charging, and the case ventilator makes charging absolutely inoffensive. The interior of the case is protected against the attack of acid by asphaltum paint.

Wires from the cells run into the special laboratory where they are connected with the distributing switchboard. This is

a 28-in. X72-in. board of black oiled slate, providing switches and plug receptacles for the control of all current which is used for any purpose within this room. The cells are connected in series groups of three each, the outside terminals of each group having double receptacles. This arrangement en

ables the operator to connect his cells with any number of groups in multiple, thus giving greater latitude in the selection of voltage and current strength than is possible with the usual series connections.

The 110-volt charging current enters the board through a switch which can be connected by plug connectors with any cell or combination of cells, and a slide-wire rheostat on the back of the board makes it possible to charge any number of cells at a time. The cells are protected during charging by an under-load circuit-breaker, and also during both charge and discharge by a fuse panel which is placed on the back of the board. An ammeter with a range of 20 amp. and a double scale voltmeter, with ranges of 150 volts and 15 volts, are provided for proper control of the charging process. The 15-volt scale is used for testing the voltage of the cells when they are not in

use.

A switch on the distributing board controls the 110-volt alternating current which is used for the lights and motors. Finally, on this board are the terminals for all of the desks, so that any operator may connect with his desk any of the cells not then in use, and in almost any combination. It will be seen that the connections on this board make the different cells and desks practically independent of each other; for instance, a part of the cells may be charging while the remainder may be distributed to the various desks as wanted.

At each working desk is a 24-in. ×36-in. slate panel which carries all of the apparatus that will be needed by the analyst, making each board an independent working unit. The voltmeter and ammeter on each board are double-scale instruments with ranges of 2 volts and 20 volts and amperes, respectively. The multiplier for the voltmeter is controlled by a small knife switch, and the shunts for the ammeter are joined to plug receptacles.

The current to each desk panel is controlled by a slide-wire taper rheostat. These rheostats are wound to give a total resistance of 130 ohms, in 254 steps. The carrying capacity of the first step is 1.3 amp. and that of the last step 25 amp., on continuous work.

For working with rotating electrodes, 1/30 h.p. alternatingcurrent, series motors of the commutator type are mounted on

the board and are controlled by a switch and 5-step rheostat. Induction motors are not used, because it is desirable to make variation in speed possible. These motors have a maximum speed of 2200 r.p.m. on 110 volts and are provided with three pulleys of different sizes.

FIG. 43. A single desk panel and rotator, Purdue University Laboratory.

In many laboratories it is the practice to mount the motor directly on the electrolyzing stand, thus avoiding all belting and making possible direct connection with the rotating electrode. On the other hand, the application of a small belt is a simple operation and not only very much decreases the vibration of

the stand, and consequently the danger of dust particles falling into the bath, but also removes the motor from the region of the bath, which in many cases contains acids and which is frequently hot. Corrosion of the motor parts is in this way largely prevented.

The stand for holding the electrodes is of iron, is quite heavy, to prevent vibration, and is fitted with rubber feet. Every portion of the base and vertical rod is heavily enameled and the electrode supports are clamped to the rod by means of heavy thumb screws, but in such a manner that the screw does not come

into contact with the rod, so that the enamel is not injured by the grip. Each clamp is insulated from the rod by a fiber bushing and each carries a binding post. There is no glass about the stand, perfect insulation of the electrode clamp being secured by the fiber bushings. The supporting ring for a dish cathode has three brass screw contacts which are adjustable for dishes of different sizes. Finally, for stationary electrodes, simpler clamps are provided to take the place of the rotator. The rotator, which carries three pulleys of different

sizes, is a vertical shaft, the lower end of which carries a universal chuck, electrical contact being insured by a brass brush.

COPPER

If a copper salt is to be used a nitrate or sulphate is best suited. Other salts of volatile acids may be converted into the sulphate by evaporating with sulphuric acid, stopping the evaporation before any decomposition into copper oxide occurs. Copper deposits in a coherent form from solutions containing sulphuric acid, nitric acid, oxalic acid and ammonium oxalate, potassium cyanide, phosphoric acid, formic acid or ammonium hydroxide. Of all of these, nitric acid produces the best results and probably sulphuric acid is next best. Chlorides should not be present. If metallic copper is to be analyzed it may be dissolved in nitric acid and the undesired excess of acid removed by evaporation.

Determination. Use enough sample to yield 0.25 to 0.50 gm of copper. Dissolve in such a manner that 200 cc of solution will contain about 2 cc concentrated nitric acid. It is sometimes desirable to make a larger quantity of solution, as 250 cc, and to use an aliquot part for each determination. In this case the acid may be added to the solution as used. The electrolysis may be begun and finished at the temperature of the room, but it will be hastened by warming the solution to about 70° at the beginning. Connect the weighed electrodes, add enough water to cover the cathode, place split cover glasses on the beaker and electrolyze with a pressure not below 1.7 volts and not above 2.0 volts unless other metals are known to be absent. The current density that may be used will depend upon the kind of electrodes. For foil cones or cylinders or for dishes, ND100= about 0.1 amp. For gauze electrodes ND100 may sometimes be as high as 5 amp. In any case the analyst must use his judgment, watching the deposited metal to discover its character. The copper should appear as a bright red metal with no spots of brown and no tendency to crumble off the cathode. When the disappearance of color indicates that the metal is deposited remove a few drops to a test tube by means of a pipette and test for traces of copper by adding a slight excess of concentrated ammonium hydroxide or of potassium ferrocyanide. The former is preferable because if copper is found the solution can be neutralized with nitric acid and returned to the beaker.

When all of the metal is found to be deposited arrange a small siphon tube in such a manner that the solution may be drawn from the bottom of the beaker without interrupting the current. As the solution is