to be a limit on the number of data collected because too much data wastes time and money. Collect a statistically viable amount of data, but be guided by common sense.

8. Keep your instruments and equipment clean. Filth cannot only chemically affect the materials of your experiment, it can also affect the operation of your measuring equipment. Therefore, just as your experimental equipment must be kept clean, it is equally important that your analysis and recording equipment also be kept clean. For example, if the mercury in a McLeod gauge or manometer is dirty, the manometer or McLeod gauge will not provide accurate readings. Similarly, if pH electrodes are not stored in a proper, standard solution, they will not provide accurate readings. Even fingerprints on objects used for weighing can affect the final weight. You are not likely to produce results worthy of note (let alone reproducible results) if your equipment is so filthy that the mess is obscuring or affecting your readings.

2.2 Length

2.2.1 The Ruler

It is appropriate that the object used to measure length is called a ruler. The dictionary's definition of a ruler is "a person who rules by authority."5 It therefore makes sense that the final arbiter of an object's length should be known as a ruler.

In the laboratory, the ruler is the meter stick. The meter stick may have inch measurements on one side (1 meter = 39 3/g inches), but a yardstick, or any ruler of 6, 12, or 18 inches has limited value in the laboratory unless it also has metric measurements in addition to English measurements.

2.2.2 How to Measure Length

Because measurements using a meter stick are straightforward, no explanation is provided. There are, however, two possible complications when using a meter stick. One problem is reading beyond the limits of the measurement device, and the other is parallax (see Fig. 2.1, Fig. 2.2, and Fig. 2.3).

Meter sticks have major limitations in that they can only measure flat objects. For example, it is impossible to accurately measure the outside diameter of a round-bottom flask, the inside diameter of an Erlenmeyer flask neck, or the depth of a test tube with a meter stick. Fortunately, there are various devices and tech-

Fig. 2.4 How to measure the inside diameter and length of an object.

niques that can be used with the meter stick. The three limitations just mentioned can be resolved with the aid of other tools, as shown in Fig. 2.5 and Fig. 2.4.

Aside from meter sticks, there are other instruments used for linear measurement such as the caliper and micrometer. Their designs and mechanisms for use are very different, but both meet specific needs and have specific capabilities.

2.2.3 The Caliper

Although the caliper is not the most commonly used measuring device used by the machining industry (the micrometer has that award), it has still tends to be limited by American industries resistance against metrics. Thus, finding a good-quality caliper with metric units may take some looking. Fortunately, the inexpensive metal calipers (with both English and metric measurements) found in a hardware store are fine for ±0.2-mm accuracy. The plastic calipers also found in hardware stores are not recommended for anything.

Fig. 2.5 How to measure the outside diameter of rounded objects.

Slide piece

(with index scale)

Outside

Fig. 2.6 The caliper and its three measurement capabilities

A good-quality caliper can provide an accurate measurement of up to ± 0.05 mm (+ 0.002 in.). For more accurate measurements [± 0.0003 mm (± 0.0001 in.)], a micrometer is used. The micrometer is discussed in Sec. 2.2.4.

The caliper has one moving part, called the slide piece, with three extensions. The extensions of the slide piece are machined so that they can make an inside, outside, or depth measurement (see Fig. 2.6). To achieve greater precision, all calipers include vernier calibration markings on their slide piece. The vernier calibration markings allow measurements one decade greater than the markings on the caliper. The length of the vernier's 10 units on the side piece are only nine units of the ruler markings on the body of the caliper (see Fig. 2.7). Fortunately, you do not need to know the geometry or mathematics of how a vernier system works to use one.

Fig. 2.7 This representation of the marks on a caliper body and slide piece shows that 10 units on the slide piece are as long as 9 units on the caliper.

The distance "X" is determined where the "index" line touches the upper scale (4.4) PLUS the number of the unit on the slide piece that best alligns with a line on the main scale (the 6th line) which provides the hundredth place measurement (0.06). Therefore "X" is 4.46 caliper units.

Fig. 2.8 How to read a vernier caliper.

This representation of the marks on a caliper body and slide piece shows that 10 units on the slide piece are as long as 9 units on the body of the caliper. A measurement reading on a caliper is made in two parts: the main unit and the tenth unit are made on the body of the caliper. The hundredth part is read on the slide piece. For example, to read the distance "X" in Fig. 2.8, first read the units to the left of the index line on the body's ruler. (The index line is after the 4.4 unit line.) Second, continue the reading to the 100th place by noting where the measurement lines of the slide piece best align with the measurement lines of the body. In this example, they align on the 6th unit of the slide piece. Therefore the 100th place is 0.06, and the total measurement of X is 4.46 units.

A dial caliper (see Fig. 2.9) is easier to read than a vernier caliper, but this ease comes at a price. An acceptable vernier caliper can be purchased for $30 - $50, and it can be found with metric and English measurements on the same tool. A good-quality dial caliper may cost between $75 and $150, and it shows only metric or English measurements. On the dial caliper, the 10th place is read on the numbers of the dial itself, and the 100th place is read on the lines between the numbers.

Fig. 2.9 The dial caliper.