Keilim, Chapter 17, Mishnah 9. This chapter has been occupied with the standard measures that halachah relies upon, and here the Mishnah turns to the most basic unit of length: the amah. Which amah do the Sages mean when they give a measurement in amot? And why were two different measuring rods kept in the Beit Hamikdash?
The unit is identified immediately. Whenever halachah speaks in amot, says the Mishnah, it has in mind the middling rod: "Ha'amah she'amru, ba'amah habeinonit." More than one length carries the title amah. There is a rod of five tefachim, a rod of six tefachim (which comes to twenty four etzba'ot, finger-widths), and a longer rod of twenty five etzba'ot. So when a measurement appears in amot with no further qualification, the intermediate length is intended, the amah composed of six tefachim.
The two rods in Shushan HaBirah
"Ushtei amot hayu beShushan HaBirah" - within the Beit Hamikdash there was a chamber known as Shushan HaBirah, and two rods, each an amah in length, were kept there. The name itself is worth pausing over. Shushan was the seat of the Persian throne, and Persia held sovereignty over the Jews settled in Eretz Yisrael during the years of the second Beit Hamikdash. When the Persian monarch authorized the rebuilding of the Temple, he asked that a likeness of Persia be displayed on the site, so that everyone who arrived would be reminded whose empire he lived under, and this request was allowed. From that likeness the chamber received its name. Both rods stored in it measured the medium amah, the length halachah normally intends.
Next the Mishnah tells us where each rod rested. One lay at the corner where the northern and eastern walls met, "keren mizrachit tzefonit"; the other at the join of the eastern and southern walls, "keren mizrachit dromit". As to their lengths: the rod in the northeast was "yeteirah al shel Moshe", longer than the amah of Moshe Rabbeinu, the accepted rod of six tefachim, by "chatzi etzba", half a finger-width. The rod in the southeast exceeded that first one by a further half etzba, so that it ended up standing a full etzba above the amah of Moshe Rabbeinu, the measure that halachah ordinarily has in view.
Why one large and one small?
Why keep two? The Mishnah puts the difficulty itself: "Velamah amru achat gedolah", why did they institute one amah that is long, "ve'achat ketanah", and one that is short? The answer follows: "Ella sheha'umanin notlin baketanah umachzirin bagedolah, kedei shelo yavo'u lidei me'ilah". The artisans engaged on the Beit Hamikdash took on their commissions as measured by the shorter rod, the one of twenty four etzba'ot, while at the time of payment the completed job was reckoned against the longer rod. In this way they could never slip into me'ilah, drawing benefit from consecrated funds by taking even the smallest fraction beyond what they had truly earned.
Picture it concretely. A craftsman contracts to build a wall at one shekel per amah. The length is measured off with the smaller rod, and it comes to one hundred standard amot, so the agreed sum is one hundred shekalim. When the work is completed, the finished wall is measured again, this time with the larger rod. Those same one hundred standard amot now register as only ninety six of the larger amot, and the craftsman is paid ninety six shekalim. He absorbs a four percent reduction from the sum he might have claimed, all in order to be scrupulously certain that not a single coin of the Beit Hamikdash's money has passed into his hand improperly.
There is a beautiful lesson embedded in these two rods. Kaspei tzibbur, public funds, demand extraordinary care. Rather than take exactly what one could arguably claim, the workers of the Beit Hamikdash accepted less, so as to remain entirely clean before Hashem and beyond any suspicion of misusing what had been consecrated.