6 ELEMENTS OF LABORATORY WORK 



For determining coincidence of liquid level with a mark on a 

 graduated vessel a reading telescope [or that of the catheto- 

 meter] is used when accuracy is needed. The surface of a 

 liquid in a tube is not horizontal. In this case the centre of 

 the surface is observed at each operation. 



Sometimes it is not possible to apply a scale directly to a 

 distance, as in measuring the diameter of a sphere. The 

 measurement then takes place indirectly. Another distance, 

 capable of being measured, is adjusted so as to be equal to it, 

 and then measured in its place. The use of callipers and 

 compass will illustrate this method, and show how the sense of 

 touch is used. Compare the results obtained with each of 

 these instruments for a given dimension, such as the diameter 

 of a sphere. The alteration of the dimensions of bodies by 

 change of temperature makes it necessary that measurement 

 should take place under the same conditions of temperature. 

 The length of a metal rod should be taken at different tem- 

 peratures to illustrate this need. 



Additional Exercises and Questions. 



1. What are the requisites of a good standard of length ? 



2. By what processes may lengths be measured ? 



3. How would you find out if a given standard is correct ? 



4. What are the advantages generally of the Metric system of 

 measurement ? 



5. Describe exactly the assumptions made in measuring any dimen- 

 sion of a body, and compare the reasoning used with that used in 

 weighing a body. 



6. Measure the distance between two given points which are not 

 connected by matter. State the precautions needed. 



7. Measure as accurately as possible by a good scale the value of 

 an inch in centimetres. Suggest methods by which greater accuracy 

 may be obtained. 



8. Use a steel scale and show that the graduations vary with the 

 temperature : measure when cold and after heating. 



9. Test the graduation of a eudiometer tube by fixing it upright 

 and reading the successive levels caused by the addition of equal 

 quantities of water. The water may be delivered from a burette or a 

 pipette. Mention the precautions which are necessary. 



