8. Describe a method by which the polarisation in an electrolytic cell can be measured. Give a general account of the phenomenon of polarisation. 9. Two small electrolytic cells, consisting of pieces of lead dipping in dilute sulphuric acid, are placed in series, and their terminals are connected to the two sides of a make and break key placed in a circuit, including a battery of e.m.f. 2 volts and an electromagnet. It is noticed that whether the key is open or shut no electrolysis takes place in the electrolytic cells, and that no spark occurs when the key is opened or shut. On disconnecting the electrolytic cells, however, a strong spark is produced on opening the key. Account for the above observations.

CHAPTER XVI

1. What is meant by "the contact difference of potential"? Give reasons for supposing that the contact difference of potential between zinc and copper is small. 2. Describe the distribution of potential in a Volta's cell when the current is (1) open, (2) closed. S. & A. D. (A.) 1893.

3. Describe the capillary electrometer, and explain how it may be employed to measure the contact difference of potential between mercury and sulphuric acid solution.

4. Describe (a) Daniell's, and (b) either Grove's or Bunsen's cells, explaining in each case the uses of the various parts. Prelim. Sci. 1890.

5. Describe the construction of Latimer Clark's standard cell. You are given five such cells (e.m.f. of each 1.45 volt), what method would you adopt to find the e.m.f. of a battery of 100 Daniells permanently connected in series? Int. Sci. (H.) 1889.

6. A Leclanché cell is connected by long thin wires to a galvanometer, the needle of which is deflected. The poles of the cell are then bridged across for a short time by a piece of thick copper wire. After the removal of the thick wire the galvanometer deflection is much less than before, but gradually rises to its former value. Explain this. S. & A. D. (A.) 1898.

7. Describe the construction of and mode of charging a secondary battery. S. & A. D. (A.) 1890.

CHAPTERS XVII AND XVIII

1. A voltaic cell is made up with zinc in a solution of zinc sulphate, separated by a porous partition from a solution of copper sulphate, in which is a copper plate. Explain where is the source of energy that keeps the current flowing when the zinc and copper are joined. B.Sc. 1889.

2. The electro-chemical equivalent of zinc is 0.00034 grams/ampere-second. Find the cost of the zinc used in a primary battery for each horse-power per hour if zinc is pence per kilo, and if the cell in which the zinc is used gives V volts (746 watts = 1 h.p.) B.Sc. 1895. Ans. .913V pence.

3. Give the theory of the voltaic battery, showing how to calculate the e.m.f. of a cell from thermo-chemical data, and how far such data are sufficient. B.Sc. 1891. 4. A cell has on open circuit an e.m.f. of 1.1 volt at o°, and when enclosed in a

calorimeter and allowed to pass a current for one hour the heat communicated to the calorimeter was 70 calories. The mean value of the current was .I ampere, and the external resistance, i.e. the resistance of the circuit outside the calorimeter, was 3 ohms. Calculate the temperature coefficient of this cell.

5. A current from a storage battery is passed through a galvanometer or amperemeter and an electric motor. Describe and give a general explanation of the difference of the readings of the galvanometer when the machine is (1) prevented from rotating, (2) allowed to run free as fast as it can. S. & A. D. (A.) 1892. 6. Give a general description of the changes which take place in the appearance of a tube, fitted with electrodes, through which an electric discharge is passed, as the pressure of the air inside the tube is decreased.

7. Give an account of the evidence for the theory that the kathode rays consist of negatively charged particles shot out from the neighbourhood of the kathode.

8. Describe experiments to show that a gas through which a discharge passes becomes dissociated.

CHAPTERS XIX AND XX

1. Give a general account, illustrated by examples, of Poynting's theory as to the transference of electro-magnetic energy.

2. Assuming Maxwell's expression for the velocity of an electro-magnetic wave, show that the refractive index of any medium must be equal to the square root of the specific inductive capacity.

3. A piece of glass, of length 10 cm., is placed near the centre of a long, thin solenoid, which is traversed by a current of 50 amperes. If there are 20 turns per cm. in the solenoid, and Verdet's constant for the glass is 0.07, find the rotation of the plane of polarisation of a beam of plane polarised yellow light produced by traversing the glass. Ans. 14° 40'.

4. Describe experiments showing that the discharge of a Leyden jar is accompanied by alternations, and state under what circumstances it becomes simply dead-beat. Illustrate your answer by reference to analogous mechanical illustrations. Int. Sci. (H.) 1892.

5. Explain how it was that Hertz was able to obtain electrical oscillations of small wave-length.

6. Give an account of some experiments on the reflection and refraction of electromagnetic waves.