Of course, my most strongly anticipated smartphone use, location services, would have been completely useless as recently as 30 years ago, let alone during the 1700s. You see, these services require at least 24 GPS satellites in near earth orbit, which didn't happen until 1993.
My smartphone's plain old telephony functionality would also have been useless surprisingly recently, courtesy of its need for large numbers of cell towers, to say nothing of the extensive communications network interconnecting them. And I am not convinced that my smartphone would have been able to use the old analog cell towers that were starting to appear in the 1980s, but even if it could, for a significant portion of life, my smartphone would have completely useless as a telephone.
Of course, the impressive social-media capabilities of my smartphone absolutely require the huge networking and server infrastructure that has been in place only within the past couple of decades.
And even though my smartphone's battery lifetime is longer than I expected, extended operation relies on the power grid, which did not exist at all until the late 1800s. So any wonderment generated by my transported-back-in-time smartphone would be of quite limited duration. But let's avoid this problem through use of a solar-array charger.
My smartphone probably does not much like water, large changes in temperature, or corrosive environments. Its dislike of adverse environmental conditions would have quickly rendered it useless in a couple of my childhood houses, to say nothing of almost all buildings in existence a couple of centuries ago. This means that long-term use would require confining my smartphone to something like a high-end library located some distance from bodies of salt water and from any uses of high-sulfur coal. This disqualifies most 1700s Western Hemisphere environments, as well as many of the larger Eastern Hemisphere cities, perhaps most famously London with its pea-soup "fogs". Environmental considerations also pose interesting questions regarding exactly how to deploy the solar array, especially during times of inclement weather.
So what could my smartphone do back in the 1700s?
- Record, store, and play back audio. This seems unimpressive, but given Dom Pedro II's "My God it talks!" reaction to a demonstration of telephony, perhaps it might pique some interest. Or provoke accusations of witchcraft, as the case might be.
- Take, store, and display photos. But it would not be possible to print them, so the services of an artist would still be required.
- Edit photos. However, this might not be seen as particularly valuable, especially given the need for an artist as printer.
- Measure sound and light intensity (courtesy of Clifford Dibble). It is not clear how valuable this would seem to our 1700s counterparts, but given the N-ray fiasco much later on, there can be no doubt that this would be quite useful.
- Act as an electronic whiteboard, albeit a rather small one. Chalkboards might be perceived as a better option, especially given that the services of a scrivener would be required to make any needed permanent records.
- Store vast volumes of information. A significant portion, and perhaps even all, of the 1700s English-language body of literature and technical works would fit into a single smartphone. "A library that fits in yor pocket!" Except that the device's long-term health would probably require it to be confined to an actual library with physical books, so perhaps it would be the searching capabilities that would be most useful. And the most threatening to 1700s men of books, which might also provoke accusations of witchcraft.
- Take, store, and display video. This could be quite impressive. However, making a permanent record would provide permanent employment for the aforementioned artist.
- Act as a small flashlight. Which is said to be the most frequent use for OLPC laptops, so this might well be the smartphone feature rated most valuable by our 1700s counterparts.
- Act as a motion sensor. I must confess that I was unable to come up with any applications of this feature that would be of any interest to my 1700s counterpart, but Maged Michael noted that it can be used as a compass and a level. Given the right apps, anyway. Perhaps given appropriate (yet to be written?) apps, this could be used for inertial navigation, perhaps for mapping caves. But given that Peter Puget was able to produce an accurate map of significant portions of Puget Sound in about a week using a pair of rowboats, some oarsmen, and standard late-1700s navigational equipment, it is not clear that smartphone intertial navigation would be seen as all that valuable.
- Tell time. But is a smartphone's timekeeping all that accurate without the occasional contact with a cell tower, GPS satellite, or Internet timeserver? If so, this would be a killer app for the Longitude Problem, but too bad about the electronics-unfriendly environmentals of 1700s ships. And the fact that there would be only one smartphone, severely limiting its usefulness.
- Human-language translation (courtesy of Clifford Dibble), at least assuming that the required translation dictionaries were downloaded ahead of time. It is quite possible that simply displaying various translation dictionaries all in a small package would be more impressive to our 1700s counterparts than the actual translation.
- Portable mathematical tables (courtesy of Clifford Dibble). Although this would likely involve a scientific-calculator app, it is likely that the best way to explain this to our 1700s counterparts would be as a very compact set of mathematical tables. The concept of a scientific calculator would likely need to come later, and perhaps much later if the 1700s counterpart in question was prone to accusations of witchcraft. (Yes, I do still have my old and well-used copy of the CRC Standard Mathematical Tables along with a rather less well-used copy of Gradshteyn and Ryzhik.)
- Act as a computer, at least assuming the requisite code-development environment was installed. This could be quite impressive, assuming that you either had an app already or snuck away somewhere in order to carry out the debugging process. I would guess that our 1700s counterparts would be thinking in terms of computing more accurate and more extensive mathematical tables. Very meticulous scriveners would of course be required for this work. Of course, this leads to really important questions, such as "What editors and computer languages would be most popular with our 1700s counterparts?"
The upshot of all this is that if my smartphone were transported back to the 1700s, it would be completely unable to provide its most commonly used 2019 functionality. However, given a solar array charger and development environment, and given a carefully controlled environment, it might nevertheless be quite impressive to our 1700s counterparts.
In fact, it would be quite impressive much more recently. Just imagine if, at the end of the Mother of all Demos, Douglas Engelbart had whipped out a ca-2019 smartphone.
But the hard cold fact is that in some ways, a 2019 smartphone would actually have been a step backwards from Engelbart's famous demo. After all, Englebart's demo allowed shared editing of a document. Lacking both wifi and a cell-phone network, and given the severe range limitations of NFC, my smartphone would be utterly incapable of shared editing of documents.
In short, although might smartphone might be recognized as a very impressive device back in the day, the hard cold fact is that it is but the tiniest tip of a huge iceberg of large-scale technology, including forests of cell towers, globe-girdling fiber-optic cables, vast warehouses stuffed with servers, and even a constellation of satellites. Thus, the old adage "No one is an island" was never more true than it is today. However, the bridges between our respective islands seem to be considerably more obscure than they were in the old days. Which sooner or later will call into question whether these bridges will be properly maintained, but that question applies all too well to a depressingly broad range of infrastructure on which we all depend.
Which leads to another old adage: "The more things change, the more they stay the same". :-)