ELEMENTS OF LABORATORY WORK 



approximate, unless comparative methods are used. The construction 

 of the frictionless wheels, however good, cannot eliminate friction. 

 Hence the rate of fall is diminished, while an equivalent rise in the 

 temperature of the apparatus takes place. A good machine, however, 

 affords a very valuable amount of training, and fair results are 

 obtained with a sufficiently long fall. The machine should be placed 

 on a very firm and horizontal bracket, fixed at as great a height as 

 the room will allow. It is essential that pulleys should be well made 

 and set, if anything like theoretical values are to be obtained Irom 

 the experiments in which they are used. The cost of such pulleys is 

 considerable. 



5. Observations of Electrification. In all observations of mutual 

 action between bodies, due to electrification, it is important that they 

 should be as completely insulated as possible. This is difficult in a 

 moist atmosphere, since the condensation of water on the surface of 

 insulating matter may convert it into a conductor. It is advisable, 

 therefore, to thoroughly clean and dry insulating stands and handles. 

 Careful washing with soda, and then with distilled water, allowing 

 the water to evaporate from the surface instead of using a cloth, is 

 generally a sufficient cleansing ; while a large copper tray containing 

 sand and heated by gas is needed for completely drying all objects 

 before use. This should be placed in a convenient position for the 

 experiment table. With electroscopes and electrometers great care 

 and patience is needed, if correct observations are to be made ; and 

 with some forms of instruments even these qualities will not lead to 

 success. Every instrument must be investigated to see that its con- 

 struction is suited in every detail to its object ; in other words, to 

 see that it really carries out what it pretends to do. The dissection 

 of instruments in this manner is an important part of scientific 

 training. 



6. Cells. The use of cells for obtaining an electric circuit entails 

 much trouble and work, unless a careful system of cleaning and 

 storing away after work is introduced. The consumable portions and 

 porous vessels should always be removed from the cells, thoroughly 

 washed, and kept always in the same place. The binding screws 

 must be kept bright, and any tendency to corrosion checked. With- 

 out rules of this kind the expenses of the laboratory will increase, 

 and, what is more important, untidy and careless work will become 

 common. A small separate room with a large shallow sink is very 

 useful for washing and storing cells, and it enables the working 

 laboratory to keep a more orderly appearance. The Leclanche cell 

 is very generally serviceable, and has the great advantages of being 



