You’ve developed a minor cold. You heard there’s something going around, maybe a virus? Or maybe it’s just an allergy. You want to play it safe and maybe get the medicines you need before the cold gets worse. So you make an appointment with a clinic for the next day. When you get there, the doctor and/or the nurse wants to get some basic details about your health first. Thus, you encounter the basic tools of a doctor visit: the thermometer, the stethoscope, the weighing scale, and the sphygmomanometer. What do they do?

What is a thermometer?

A thermometer is a device to measure the temperature of an object. Every thermometer has two parts: one part is sensitive to changes in temperature and the other part shows these changes as numbers.

For example, in the once-ubiquitous mercury thermometer, a small volume of mercury in a glass bulb is the temperature sensor and its rise and fall in response to temperature changes happens through a glass capillary marked with numerical values. When the mercury bulb is brought in contact with a warmer object, the liquid expands and rises up through the capillary. The number next to its uppermost level shows the temperature of the object.

This relationship between the temperature and the mercury’s level requires each mercury thermometer to be carefully calibrated such that the capillary’s width, the mercury’s ‘base’ temperature, and the markings on the scale carefully match each other. It’s also important that the material that fills the bulb — mercury in this case — has a linear dependence on temperature changes: that is, if it expands by 1 unit for every 1º C change in temperature, it should do so by 10 units for a 10º C change or by 0.1 units for every 0.1º C change.

Researchers have developed more advanced thermometers today that use digital components to infer the temperature. For example, thermal guns of the sort used to check the temperatures of people entering malls and supermarkets during the COVID-19 pandemic contains a detector that interprets the radiant power of a warm body as changes in the voltage or resistance in a circuit, and uses that to calculate a temperature readout.

Another common type of thermometer is the thermistor: a semiconductor whose resistance is highly sensitive to changes in temperature; thermistor-based thermometers are today a common sight in clinics and hospitals.

What is a stethoscope?

The stethoscope is a simple mechanical device. It consists of a diaphragm at one end and an earpiece at the other, with a tube connecting the two. When the diaphragm is placed against the skin, sounds inside the body near that area cause the diaphragm to vibrate, creating pressure waves in the air filling the tube that are transmitted via the earpiece to the listener.

The piece that holds the diaphragm has a bell-shaped opposite side. The diaphragm is suited to pick up on high-frequency sounds while the bell is suited for low-frequency ones. As with most other medical devices, stethoscopes also have electronic versions with additional features. These are sometimes called stethophones.

A particularly important feature is sound amplification, to help the listener listen to sounds too faint to be transmitted in an acoustic stethoscope. Some companies have developed stethophones that can collect sound data, analyse it for signs of certain heart conditions, and transmit the results wirelessly to a smartphone. A few products even allow the stethophone to acquire an electrocardiogram of the heart.

How do weighing scales work?

Perhaps the simplest of (today’s) weighing scales is the spring scale, a common sight in clinics, house bathrooms, and grocery stores.

A quick primer first: the mass of an object is an implicit property, the amount of matter the body possesses, while its weight is the force exerted by gravity on that object. So a body has mass whether or not a force is acting on it.

Back to the spring scale: This device consists of a spring. That’s pretty much it. When the spring is placed under a plate and an object is placed on top of the plate, the device is calibrated to measure the force exerted by the object based on how much the spring is compressed. Alternatively, the spring can be attached to a hook at the bottom from which an object can be suspended, the object’s force calculated based on how much the spring is stretched. The principle is the same.

An operator just needs to make sure the spring works as expected at regular intervals and to calibrate the weighing scale at the location where it’s going to be used, rather than somewhere else, to account for local variations in gravity.

An electronic weighing scale works the same way except for the part where the spring’s compression or stretching is converted to a weight measure. An analog device does this by calibrating the spring before it’s installed in the scale. An electronic device uses a force transducer for this task. Two common ones are load cells and strain gauges. A load cell is a type of circuit that, when a voltage is applied, transmits different amounts of current depending on the weight acting on it. A strain gauge is a conductor whose resistance changes depending on its length.

What is a sphygmomanometer?

A sphygmomanometer is a device that monitors blood pressure. Its name has Greek and French roots that together mean “pulse pressure monitor”. A common version of the device is manually operated. It consists of an inflatable cuff and a manometer, a device that measures pressure. The cuff is strapped around an individual’s arm, at roughly the same height as the heart. The pressure inside the cuff is increased by pumping a bladder of air. The doctor or nurse keeps track of the cuff pressure by its link to the manometer, which measures pressure as the level of mercury in a capillary (like in a mercury thermometer). The worker also has a stethoscope placed over the brachial artery, the main artery bringing blood to the upper arm.

When the pressure in the cuff reaches a suitably high value, the artery will be pinched off and blood flow will stop. The worker then slowly releases the pressure in the cuff, at about 2 mm of mercury per heartbeat. When the pressure in the artery drops below the threshold that allows blood to flow, a whooshing sound can be heard via the stethoscope. The manometer reading at this point is the systolic pressure. When the whooshing sound stops altogether, the manometer reading is the diastolic pressure.

The whooshing is called the Korotkov sound, for the Soviet physician Nikolai Korotkov, who discovered it in 1905.

Electronic sphygmomanometers automate this process but with a slight difference. When the pressure in the cuff is neither too high to stop blood flow nor too low to let it flow unimpeded, and partly restricts flow, the cuff pressure will oscillate over a small range in sync with the expansion and contraction of the artery. The device uses this oscillation to calculate blood pressure.

Electronic sphygmomanometers require less skill to operate and can even be installed for home use, but they are also less accurate than the mechanical, mercury-based version, which are preferred during clinical trials and in low-key clinical settings. In particular, electronic devices fail to provide faithful readings when the individual has a pulse-related abnormality like pulsus paradoxus or arrhythmia.

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You take the data about your weight, BP, and body temperature to the doctor, who becomes aware of your general condition – all in a matter of minutes. You also describe your cold symptoms and medical history. She pulls out her stethoscope and begins the consultation with a good idea of what’s going on…