5'J DR. F. HORTON ON THE EFFECTS OF CHANGES OF TEMPERATURE 



The arrangements for holding the wire are best seen in fig. 6. One end, A, is firmly 

 clamped and the other is attached at B to a vertical lever CBD, of which the fulcrum 

 is at C. A string is attached to the lever at D, and this passes down the grooved 

 side of the metal piece EFG (which serves the same purpose as a frictiouless pulley- 

 wheel) and carries at its lower end the mass H. This mass is so chosen that the 

 tension produced in the wire is the same as that due to the weight of the vibrator in 

 the rigidity experiments. 



The wire is surrounded by water except at the points the distance apart of which is 

 to be measured. At these points it passes transversely through small brass tubes 

 closed at the lower ends. These tubes (U and V, fig. 6) are supported in a vertical 

 position by wires from the base bar L. The holes in the tubes were made the same 

 size as the cross-section of the wire and just outside, both wire and tube were covered 

 with melted shellac, which effectually prevented any water leaking in. The standard 

 thermometer T was supported beside the wire by means of supports from the base 

 plate L. 



The water in the trough can be heated by means of the row of burners RR, shown 

 in fig. 4. The gas feeding these passes through a gas regulator K, the thermal part 

 of which is a mercury bulb, 1 '3 centims. in diameter, runninsr the whole length of the 

 trough on the opposite side of the central partition to the wire. With this heating 

 and stirring arrangement the water in the trough can be kept at a temperature not 

 varying by more than '05 C. for hours. 



The top of the trough was covered over with pieces of bright tin plate, a small 

 circular hole being left under each of the microscopes. These holes were also covered 

 over except when readings were being taken. The thermometer was read to '01 C. 

 by means of a microscope, the covering of the trough above the part of the stem 

 which was required to be seen being removed for this purpose. In order that the 

 rocking plates and objectives of the microscopes might not be clouded with steam at- 

 the higher temperatures, a layer of ordinary lubricating oil was poured over the surface 

 of the water. This effectually prevented any steam escaping, and also helped in 

 maintaining the temperature constant. The microscopes were arranged on the slab 

 so that each was over the corresponding tube on the wire, and particular care was 

 taken to see that the wire was horizontal and parallel to the direction of motion of 

 the trough. The height of the second trough containing the standard metre was 

 adjusted so that the scale could be seen clearly without re-focusing the microscopes. 



The difficulty in measuring small alterations in length directly by means of 

 a microscope is to obtain a sufficiently fine fiducial mark. This difficulty is especially 

 great in the case of a wire, on which it is very difficult to make a fine mark. After 

 many failures it was found that the best plan was to ma}) out the appearance of the 

 surface of the wire and fix on some easily distinguishable point. The illumination 

 was secured by an excellent arrangement designed by Professor POYNTING. By 

 means of this the light from a small 10-volt electric lamp, enclosed at the end of 



