XII. SPECIAL COLLECTIONS. 337 



19. Plate lightning protector for telegraph stations. The line plate and 

 earth plate are of brass, transversely grooved and mounted on marble slab. 



20. Lightning protector for telegraph stations. A fine silk covered wire 

 forms part of the line circuit, and is wound round a grooved copper cylinder 

 which is to be placed in connexion with earth. 



21. Tubular lightning protector for telegraph stations. The line tube and 

 earth tube are transversely grooved. 



22. Universal galvanometer (portable), which serves for the following 

 purposes : 



1st. Measuring electrical resistances, the instrument being arranged as a 

 bridge : 



2nd. If or comparing electromotive forces. Professor E. du Bois Reymond's 

 modification of Poggendorff's compensation method being used : 



3rd. For measuring the intensity of a current, the instrument being simply 

 used as a siue galvanometer. 



The instrument consists of a sensitive galvanometer which can be turned in 

 a horizontal plane, combined with a resistance bridge (the wire of which 

 bridge instead of being straight is stretched round part of a circle). The 

 galvanometer has an astatic needle suspended by a cocoon fibre, and a flat 

 bobbin frame wound with fine wire. The needle swings above a cardboard 

 dial divided in degrees ; as, however, when using the instrument the deflection 

 of the needle is never read off, but the needle instead always brought to zero, 

 two ivory limiting pins are placed at about 20 degrees on each side of zero. 



The galvanometer is fixed on a graduated slate disc, round which the 

 platinum wire is stretched. Underneath the slate disc three resistance coils 

 of the value of 10, 100, and 1,000 Siemens' units are wound on a hollow 

 wooden block which protrudes at one side; this projection carries terminals 

 for the reception of the leading wires from the battery and unknown resistance. 

 The adoption of three different resistance coils enables the measuring of large, 

 as well as small resistances, with sufficient accuracy. 



The whole instrument is mounted on a wooden disc which is supported by 

 three levelling screws, so that it may be turned round its axle. On the same 

 axle a lever is placed which bears at its end an upright arm, carrying a con- 

 tact roller. This roller is pressed against the platinum wire round the edge 

 of the slate disc by means of a spring acting on the upright arm, and forms 

 the junction between the A and B resistance of a Wheatstone's bridge, which 

 resistances are formed by the platinum wire on either side of the contact 

 roller, one of the three resistance coils forming the third resistance of the 

 bridge. 



23. Sine galvanometer (portable). The magnetic needle moves on a steel 

 point within the wire coil, and graduated segment inside a circular brass case 

 which turns in a graduated circle. 



24. Sine galvanometer (portable) with 3 coils. 



25. Detector galvanometer with oscillating magnet. 



26. Differential galvanometer, constructed on the double shunt principle. 

 27- Sounder telegraph instrument with split armature. 



28. Set of ink recording instruments for duplex telegraphy, according to 

 the differential system. 



29. Double current Morse key with automatic switch for sending and re- 

 ceiving position, to be used with a single battery. 



30. Morse key for testing purposes. 



31. Double current Morse key for testing purposes. 



32. Double current Morse key for signalling. 



33. Discharging key. 



34. Magneto induction bell with inductor of 6 magnets, for signalling on 

 railways, mines, inclines, &c. without the use of batteries. 



39508. Y 



