Measuring the same sample should give the same result every time if the equipment is precise. In practice, the information displayed by a measuring device can depend on several factors (such as temperature and humidity) and can drift slightly over time. Nevertheless, during the time it takes to complete a measurement sequence, all measurements ought to remain within a specified, small margin of error, often marked on the equipment. We will see later on, in Author(s): The Open University

The way to ensure that equipment is accurate is to use a series of known standards against which to calibrate the equipment. Calibrating should be done at least each day and sometimes more frequently (such as before using the equipment to measure unknown samples). Many types of measuring equipment go through an automatic calibration when they are switched on, but others require the user to provide a series of known calibration standards.

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Subtraction of numbers can be used to answer questions such as â€˜what's the difference between two values?â€™ or â€˜if something has decreased by a certain amount, what's its new value?â€™ Subtraction can also be thought of as undoing the process of addition. For instance, instead of saying â€˜Â£10 take away Â£7.85 leaves how much?â€™ you could say, â€˜what do I have to add to Â£7.85 to get back to Â£10?â€™

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If we add 109.8 ml of one liquid to 6.5 ml of another liquid, what would be the total volume of liquid in ml?

To compare 109.8 with 6.5, you need to remember that

Place the two numbers in a grid on top of each other and make sure that columns representing the same magnitude line up wit
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If you have difficulty with this section, you might find it helpful to investigate some of the Government schemes aimed at improving maths skills. More information about such schemes can be found at http://www.direct.gov.uk/en/EducationAndLearning/AdultLearning/ImprovingYourSkills/index.htm (accessed 5 March 2008).

## Box 3: The basicsAuthor(s): The Open UniversityLicense informationRelated contentExcept for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

Two of the frames show Al Bean carrying the Apollo Lunar Surface Experiments Package (ALSEP) packages out from the Lunar Module. (QuickTime, 500KB, note: this may take some time to download depending on your connection speed)

This unit is an adapted extract from the course Biological psychology: exploring the brain (SD226)

This unit looks at how language is understood, which includes hearing and how sounds and words are interpreted by the brain. It takes an interdisciplinary approach and should be of wide general interest.

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The document attached below includes the third section of Mountain building in Scotland. In this section, you will find the following subsections:

• 3.1 Introduction

• 3.2 Palaeoproterozoic rifting, sedimentation and magmatism

• 3.3 The Palaeoproterozoic Laxfordian Orogeny

• 3.3.1 Assembly of the Lewisian Complex

• 3.3.2 Formation of Proterozoic crust

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The document attached below includes the table of contents and first section of Mountain building in Scotland. In this section, you will find the following subsections:

• 1.1 Setting the scene

• 1.2 Recognizing ancient mountains

• 1.3 Orogeny through geological time

• 1.3.1 Geological time: a brief note

• 1.3.2 Disentangling the cont
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Except for third party materials and otherwise stated (see terms and conditions), this content is made available under a Creative Commons Attribution-NonCommercial-ShareAlike 2.0 Licence

Grateful acknowledgement is made to the following sources for permission to reproduce material in this unit:

## Author(s): The Open UniversityLicense informationRelated contentExcept for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

Psychophysics is the oldest field of the science of psychology. It stems from attempts in the nineteenth century to measure and quantify sensation. It attempts to quantify the relationship between a stimulus and the sensation it evokes, usually for the purpose of understanding the process of perception. Historically, psychophysics has centred around three general approaches. The first involves measuring the smallest value of some stimulus that a listener can detect â€“ a measure of sensitivit
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Information about stimulus intensity is encoded in two ways: the firing rates of neurons and the number of active neurons.

Intensity is assumed to be encoded by an increase in discharge rate of action potentials within the auditory system. As the stimulus gets more intense, the basilar membrane vibrates at a greater amplitude causing the membrane potential of activated hair cells to be more depolarised and this causes the nerve fibres that synapse onto the hair cells to fire at a greate
Author(s): The Open University

It is believed that tip links aid in causing â€˜channelsâ€™ to open and close near the top of the hair cell (Figure 16). Tip links are filamentous connections between two stereocilia. Each tip link is a fine fibre obliquely joining the distal end of one stereocilium to the side of the longest adjacent process. It is thought that each l
Author(s): The Open University