MEASUREMENTS 



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torn of the curve will be easily identified. If the eye is placed shghtly 

 below the graduation line, the graduation ring looks like an ellipse. 

 Placing the bottom of the meniscus at the center of this ellipse gives a 

 precise reading and avoids errors as 

 great as the thickness of the gradua- 

 tion line. 



A pipette is really a miniature bu- 

 rette, consisting of a glass tube with 

 one or more graduations engraved 

 along its length. Measured quantities 

 of liquid are transferred from one 

 container to another by sucking up a 

 pipetteful and then discharging the 

 amount of liquid designated by the 

 scale marks. A volumetric or transfer 

 pipette (Fig. 4-4a), designed for 

 transferring certain exact volumes of 

 liquid, has only one mark on the tube. 

 The pipette is filled to about 15 to 

 20 mm above the line, and then held 

 vertically with the index finger on 

 top controlling the flow of liquid. 

 (Some beginners seem to prefer to 

 use a thumb, but this is about as 

 awkward as holding a fork as you 

 would a hammer.) A slight rolling 

 motion of the finger allows perfect 

 control, and the excess liquid is 

 drained off down to the mark. The 



liquid is then allowed to flow out at Measuring, (c) Serological, 

 an unrestricted rate until the level has 



reached the bottom of the bulb of the pipette. Finally the tip is held 

 against the wet side of the vessel until the liquid has stopped flowing. 

 The last portion of a drop is ordinarily not blown out of a volumetric 

 pipette. 



Measuring pipettes (Fig. 4-4b) are graduated in milliliters, with 

 at the top. The full capacity is contained between this mark and a mark 

 near the bottom. The liquid is allowed to drain as rapidly as possible 

 and still retain enough control to stop at the desired point. When the 

 full quantity has been "delivered," touch the tip to the side of the 



V 



(o) 



V 



(b) 



= 1 



.12 





(c) 



Fig. 4-4. Three kinds of pipettes. 

 (a) Volumetric or transfer, (b) 



