Alphabet’s health spin-off Verily is a little like Santa’s factory a month before Christmas. Its labs are full of promising ideas not quite ready for delivery. These include a glucose-sensing contact lens, a cancer-detecting wrist band, and a big study of what it means to be healthy.

However, during a visit to Mountain View-based Verily last week, I became the first journalist (that I know of) to see a prototype of the company’s health-tracking watch.

Not only is the watch real, but Verily has built “more than hundreds” of them, according to Brian Otis, who is the health company’s chief technical officer and an expert in low-power electronics from the University of Washington.

This is not to be confused with the actual Verily watch, which unfortunately was not made available to us to photograph.

Otis, who was wearing the watch, let me inspect it briefly during an interview on a different matter. “I don’t want to talk about the technical details a lot,” he said, taking the watch back.

Verily is a spin-off of the experimental “X” division of the search giant Google, which renamed itself Alphabet a year ago to reflect its widening ambitions, which now include selling home gadgets, as well as developing self-driving cars and a variety of medical devices.

The watch could be a big deal for Verily, which is promising to combine “sensors, software, and science” to come up with new ways to measure disease and help control it.

In June 2015, Bloomberg first reported that Google’s life sciences spin-off had created “a health-tracking wristband” for use by medical researchers to get “minute-by-minute data on how patients are faring” and that it would measure pulse, heart rhythm, skin temperature, light, and noise.

The prototype I saw was set in an ordinary-looking brass-colored analog watch casing that appeared not to have any buttons. Otis called it the “Cardiac and Activity Monitor” and said it was at least the second generation of the device.

A couple of novelties jumped out. The first was the display, a paper-white circular dial with a simple digital time readout. It uses e-ink, a display type that uses less electricity. “If people are going to wear this you can’t charge it every day; that just isn’t going to work,” says Otis, whose watch featured a pink band made of flexible rubber. “The big push now is low power.”

The plain display is a sign that this device exists to collect data, not show it to users. In fact, Verily has said it’s not a consumer gadget at all. Instead, it plans to use the device in medical research such as its Baseline Study, a large-scale project it says will measure and track thousands of individuals in order to look for new signals—in their blood or from wearable monitors—that can predict disease.

At Verily’s headquarters, half-completed watches were being assembled at a work bench, reflecting the company’s effort to build new devices from the ground up, including designing the chips that go inside them. “We are constantly iterating on these things,” says Otis.

An outer ring on the prototype, Verily confirmed, measures a person’s electrocardiogram (ECG), or the electrical rhythm of the heart. Because such a measurement usually requires making electrical contact with two distant points on the body, it may be that users grip the ring with their other hand. Verily didn’t say, nor has it said if the measurements are accurate. A Verily spokesperson declined to provide detailed specifications on the watch, saying it was still under development.

Even so, turning over the watch offered a quick peek at its other medical sensors. Four optical elements appeared to include two green LEDs. Flashing such lights into the skin is a means of measuring heart rate (also employed by the Apple Watch) called a “photoplethysmogram.” It relies on the fact that blood absorbs green light, and will absorb more the harder your heart is beating.

The underside of the watch also featured four raised metal pads, worn tightly against the wrist. These may have several uses, including as contacts to charge the watch, or to provide a second electrode to complete the ECG measurement.

David Albert, founder of AliveCor, which markets a portable ECG monitor that works with the iPhone and has developed an ECG watchband, says these elements could also be electrodes to measure how much tissue resists a small electrical current. That is another way to measure heart rate, also employed by Jawbone Up wristbands, and one which uses less power.

Another possible use for such a setup, says Albert, would be to measure galvanic skin response, or how much you sweat, and a measure of stress. The watch also contains an accelerometer and a gyroscope for measuring movements, which are found in many fitness wearables today.

It’s likely that Verily is seeking to master new measurements, such as detecting blood pressure, though perhaps not using the prototype I saw. The Alphabet division recently hired David He, a wearables innovator who attempted to build a blood-pressure watch at the startup Quanttus, in Cambridge, Massachusetts, before that project ran into difficulty.

Dennis Ausiello, a senior physician at the Massachusetts General Hospital and a paid scientific adviser to Verily and its CEO, Andrew Conrad, says “there’s nothing magic about a watch” except that people are relatively likely to wear one.

The watch or technologies like it could find use in the Baseline study, which Ausiello says might eventually involve 10,000 to 20,000 people. Verily says it plans to describe Baseline in more detail later this year, but it appears to be a large epidemiological study, meaning it will try to discern new patterns including discovering biomarkers able to predict who gets sick. Ausiello says a key goal is to use measurement tools that are “minimally invasive and minimally intrusive” to collect large amounts of data from volunteers on an ongoing basis.

“The watch is one of several hardware activities that have a common goal, which is how to better manage the human condition and interrogate the human organism at scale across health and illness,” he says. Verily says it has not settled on the final lineup of sensors or instruments it would use in the study.

Making a medical-grade health watch hasn’t been easy. Apple scrapped a number of health measurements shortly before the launch of its Apple Watch partly because they were unreliable. Albert says one problem is that for more sophisticated measurements with a watch or band “you have to wear them very tightly on your wrist and a lot of people don’t like that. And if you leave them on your dresser or desk they don’t work at all.”