Scientists can measure atoms and they can measure the distance from our sun to the edges of the universe. So of course they also have instruments that can determine precisely how sexually aroused you are.

Most of these devices are used in research labs, but a few are for the amusement and edification of consumers.

John Perry, inventor of this "insertable device" for measuring female sexual arousal, explains:

The combination blood-flow sensor ("photoplethysmograph") and muscle activity sensor ("electromyograph") was first developed to investigate the mechanisms of sexual arousal. Along the lower edge of the sensor barrel a single longitudinal silver EMG electrode is visible. (Two other long silver electrodes are located at 90 and 180 degrees behind the sensor body.) Above the electrode, the dark circle of a photocell aimed at the vaginal wall is visible. To the left of the photocell, the white base of a miniature incandescent lamp is visible. A five-pin DIN plug was molded into the base. An insertion stop, right, prevents the sensor from going too far into the vagina, and a retaining bulb, left, helps to prevent it from falling out during contractions.

Things have gotten a bit more high tech since Perry was playing around with his combination photoplethysmograph and electromyograph. A newer model is preferred by researchers at the University of British Columbia's Sexual Psychophysiology and Psychoneuroendocrinology Laboratory. On their lab website, they explain their next-generation photoplethysmograph:

A small, tampon-shaped device which is self-inserted into the vagina and measures Vaginal Blood Volume, Vaginal Pulse Amplitude, and heart rate in response to an erotic stimulus. The vaginal photoplethysmograph is completely safe and is sterilized in Cydex-activated glutaraldehyde. This sterilizing procedure is commonly used with hospital instruments and is known to prevent both viral and bacterial transmission of infection. Participants insert the device in the privacy of a locked room.

I like that they mention how it's sterilized between each use. I should hope so!

But a lot of women might want something a little more intimate to measure their arousal. None of these "tampon-shaped devices" that contain electrodes and lights that measure blood flow. For those women, Laura Lanzo has recently invented lingerie covered in heat-sensitive beads (like mood rings) that change color as the temperature of various erogenous zones increases. Lanzo's lingerie is working on the same principle as the photoplethysmograph, which measures increased blood flow - and therefore increased heat - to the genitals. Plus, one hopes that the final result will look more enticing than this patent diagram. Here's how Lanzo describes her creation:

Temperature-sensitive mood stones or beads are incorporated into lingerie at positions that indicate sexual arousal to indicate by observation the state of arousal of the individual wearing the lingerie. In one embodiment, the mood beads are located adjacent the groin area for women's panties and at various erogenous zones of a brassiere, including cups and straps.

So we've got the measurement of women's arousal (sort of) converted into raw data. Now what if you're a lad and would like to know more about your erections?

Here's the "penile cuff." This device measures blood flow to the penis, which is to say the temerity of your tumescence. It was invented by a group of researchers in France, who report on how it works in the Nature's International Journal of Impotence Research:

Fitted on to the penis shaft, it is connected by a pressure tube to a three-way tap, one outlet of which is connected to a 5 l perfusion bag held 30 cm above the penis so as to maintain constant pressure in the cuff whatever the penis volume. The third tap outlet is connected to a pressure gauge, which monitors the stability of the pressure, and to a latex membrane the pulse volume linked movements of which are recorded on a photoplethysmograph. (2) Penile cuff number 2 is a tourniquet enabling cavernous artery flow to be varied by means of a water-filled syringe. (3) Flow variations caused by the tourniquet were recorded simultaneously by continuous Doppler velocimetry and PVP. (4) Both signals were digitized for subsequent computer comparison.

You probably can't get one of these, but if you want to do similar kinds of monitoring on your own, a Windows-compatible USB device is available from SMT Medical that does photoplethysmography and pulse wave velocity measurements using a nice little cuff.

Penile plethysmography is often used in the treatment of sex offenders. Clinicians show the offenders "inappropriate" pictures and measure to see if the guys are popping wood or not. There have been a number of attempts to introduce measurements from these devices in court, essentially to prove somebody is a sex offender (or potential sex offender). But most courts have deemed them inadmissible.

Another way that researchers measure male arousal is by monitoring muscle contractions during orgasm. The best way to do that is to insert an anal probe that measures muscle contractions while a man masturbates to orgasm. A group of researchers at the University of Minnesota Medical School conducted extensive research into male orgasms using the anal probe, and published the results in The Archives of Sexual Behavior. It's worth quoting at length from their study. Here's what they said:

Pelvic muscle contractions during sexual response can be monitored conveniently by the anal probe method described. Eleven young adult male subjects were each recorded for three sessions of masturbation to orgasm. Electrical signals from an anal pressure probe were automatically digitized by computer. Orgasmic contractions were easily distinguished from voluntary contractions by the steadily increasing intervals and complete muscle relaxation between orgasmic contractions. At orgasm each subject produced a characteristic series of contractions starting abruptly at an intercontraction interval of about 0.6 seconds, and continued for 10 to 15 contractions at an increasing increment of about 0.1 second per contraction. Pressure amplitude, representing the force of contractions, increased from the beginning of the regular series to a maximum at the seventh or eighth contraction. Area under the pressure curve, reflecting muscular exertion during contraction, generally increased throughout the regular series. Each man's pattern of contractions was very similar from one session to the next and distinguished his records from others'. Individuals' patterns could be grouped into three types, based chiefly on the location of the regular contraction series within the subjective span of orgasm. The most common type was a simple series of regular contractions. It had the shortest duration and fewest contractions. The next most common pattern began with the regular series, followed by a number of irregular contractions. This type was longest in duration. One man with a third type, of intermediate duration, had a number of preliminary contractions before the series of regular contractions began in midorgasm.

I'll bet you didn't know the male orgasm was so precise, or so varied! You can get your own anal pressure measurement device here (pictured at left, above). Or you can get the EMG anal probe here (pictured at right).

There are other wonderful devices for measuring penile activity as well. Take a gander at this device for inducing and measuring premature ejaculation. Essentially it's a vibrator attached to a stopwatch. The person who owns the patent on this device (yes, it's a patented device) points out that premature ejaculators can use the device to measure their progress towards having, um, mature ejaculations. Honestly, isn't this just an excuse to have a fancy male vibrator?

Then of course there are the no-excuses vibrators. The "smart sex toy" (below), invented by Californian Gregg S. Homer, uses sexual arousal monitoring devices to determine how excited users are - then passes that information into a laptop (see diagram) and passes stimulation on to another partner who is also hooked up to the laptop. So you've got sexual arousal monitoring for the input, and sexual arousal inducing devices as your output. What could go wrong? Homer explains his device to the patent office as:

A system of masturbation comprising: (a) means for collecting biological data from the user; (b) means for receiving and processing the biological data; and (c) means for delivering sensual stimulation to the user in response to the biological data.

Ah yes, the old "system of masturbation." Notice that he doesn't explain much about what those monitors and sensual stimulators might be. Use your imagination! That's what the patent officers who approved this device did.