Instruments and Accessories
2.2 TRAVELLING MICROSCOPE
The travelling microscope [Fig. 2.4 (a)] is an instrument well- suited for the purpose of measuring small vertical or horizontal distances with high accuracy. It consists of a compound micro- scope that is capable of independent horizontal and vertical movements. The amount of movement in the vertical direction can be obtained from the scale M1 and the vernier V1 where as
that in the horizontal direction can be obtained the scale M2 and
the vernier V2. The microscope may be raised or lowered along
a vertical pillar PN and its axis may be fixed horizontally, vertically, or in between them by the screw S1. The screw S2 is
used to move the pillar PN horizontally. The object is viewed through the eye piece E when the objective lens O is turned towards the object. The focal length of the objective generally lies between 3 to 4 cm. The focussing of the microscope is
accomplished by a screw attached to the body of the microscope. The cross-wire of the eye piece is focussed by moving eye piece in or out. Four screws at the base of instrument are used for its levelling.
Before using the instrument note the vernier constants of the both the verniers V1 and V2.
Generally the vernier constants of V1 and V2 are the same.
The procedure to measure the horizontal or the vertical distance between two points is as follows:
(i) Level the instrument with the help of the base screw and a spirit level.
(ii) View one of the points through the microscope and focus the cross-wire with the image of the point. Note the readings of the main scale and the vernier scale (v.s.).
(iii) Displace the microscope vertically or horizontally, as required by means of the screw
S1 or S2to view the second point and focus the cross wire with the image of the second point. Note again the reading of the main scale and the vernier scale.
(iv) Calculate the difference (horizontal or vertical) between these two readings to obtain the distance between the points.
2.3 CATHETOMETER
This instrument is used to measure vertical distances accurately in experiments where the range of a travelling microscope is inadequate. A typical form of the instrument is shown in Fig. 2.4(b).
It consists of a graduated rod AA¢ held vertically by means of a stand provided with three levelling screws (S1, S2 and S3) at the base. The rod AA¢ carries a slide to which a telescope T is mounted horizontally. The rod along with the telescope can be rotated about a vertical axis. The screw F is used for horizontal adjustment of the telescope. A spirit level L is mounted on the top of the telescope to indicate the horizontal condition of the axis of the telescope.
The slide along with the telescope can be moved vertically up and down and can be fixed at any position by means of a screw S. The carriage holding the telescope has a vernier V. This vernier along with a micro meter screw M is used for measuring the small vertical distance. To measure a vertical distance with the help of a cathetometer adopt the following procedure:
(i) Make the telescope parallel to the line joining any two of the levelling screws of the stand. (Some cathetometer are pro- vided with two adjustement screw and a pin of fixed length is fixed to the third leg. For such cathetometers the telescope is set parallel to a line that passes through the fixed pin and any one of the two screws.) If the bubble of the spirit level is away from the centre of its scale then bring it half way back to the centre by turning the screw F.
(ii) Now turn the base screws simultaneously by equal amounts in opposite directions until the bubble reaches the centre of the scale. Note that these screws are in a line which is parallel to the telescope axis. (For those having two adjustable screws and a fixed pin, adjust the screw that in conjunction with the pin forms an imaginary line parallel to the axis of the tele- scope).
(iii) Turn the rod AA¢ through 180°. If the bubble of the spirit level is found away from the centre, bring it to the centre by first adjusting the screw F and then the two base screws, as dis- cussed in steps (i) and (ii).
(iv) Turn the rod AA¢ again through 90° so that the telescope is now set at right angles to the previous line joining the two base screws. Adjust the remaining base screw and bring the bubble back to the centre, if necessary.
In practice, the above four steps are to be repeated several times in order that the bubble remains at the centre of its scale irrespective of the direction in which the telescope is set.
To measure accurately the vertical distance between two points very close to each other, focus the telescope on the first point until the image of the point coincides with the cross-wires of the telescope. Note the vernier reading. Alter the vertical position of the telescope and focus on the second point so that its image coincides with the cross-wires. Again note the vernier reading. The difference between the first and the second vernier readings gives the vertical distance between the points.
A very familiar application of cathetometer in the laboratory in the measurement of the vertical depression of the mid-point of a bar place horizontally between two knife edges and loaded at the middle.